As I understood, you want to be able to expand your setup in the future, but you're currently limited by your budget. That's not an easy starting point, but let's brainstorm and work on the process of elimination to figure out what you perhaps avoid.

If you plan to expand later on, you should choose equipment with expandability in mind. Buying something too narrow just to fit your current budget doesn't make sense because you'll lose money when you buy and sell again. While RME devices hold their value better due to quality and long-term driver support, if your budget is tight, it's still best to avoid this if possible.

Furthermore, consider the time factor. If you buy a different device later on, you'll have to redo the entire TotalMix configuration. If that's not something you're good at or haven't documented well, it could mean a lot of work.
Time will be lost, time you could spend with your family or earning money (what also could be used to finance the setup).

So, you should aim to buy something that offers investment security for a longer time range or even features that make your work easier or that you consider important for your setup or workflow (DURec, Autoset, etc.). An UFX III could, e.g., be the recording interface for your lifetime (due to MADI expandability and the long driver support of RME, for some products already 20+ years).

Depending on the terms, I wouldn't rule out installment payments, either. You'll have to calculate that. Maybe pay €1500 upfront and then the rest in installments. You could also use your current 802 for trade-in value, as it might turn out that you're better off aiming for UFX II or even UFX III (due to better expansion capabilities with MADI).

While an 802 is already a great interface, what I, personally, don't like about it compared to flagship interfaces is that certain level settings (microphone, headphones) can only be controlled analogously via knobs. I know there are still enthusiasts for this, but I find it much better if these values can also be digitally stored and recalled via TotalMix FX snapshots and workspace, so you can recall a specific setup (especially in a home/project studio) easily. It saves time. I also miss features like Autoset for quicker level adjustments for microphones or connected instruments (guitar, bass, etc.). Or DURec for backup recordings or standalone recordings. The operation or display options already provide a concrete added value. A quick glance at the display and I can see if something is clipping, without having to bring up the TotalMix FX window separately (I only have Full-HD resolution).

Ok, the new 802 FS has the better analog and digital section of UFX II and UFX III. But I would definitely consider whether it's worth it (since you're already investing so much). I think it's better to skip it and to go for the UFX II or UFX III because of certain features, some of which I've mentioned above, but there are also others.

So, now you're in a price range of a UFX II or UFX III, which ranges between €2099 and €2689. ARC USB is also recommended (€159), but you can also buy it later (a nice Christmas present).

In this price range, you probably won't consider adding an ADI-2 ***, as the quality of the UFX III is already fantastic, and you do not need more. Here, I would really focus and trust on the features and expandability of an UFX III. You can always use or need a few more mic preamps or analog channels in the future.

With ADAT, you can connect 2×8=16 channels at single speed (44.1-48 kHz), but then the device is full. If you ever want to record at double speed, you'll only have 2×4=8 channels available, so you'd have to disconnect either the preamp or AD/DA converter and plug in the 2nd ADAT connection. But you'd like to avoid such rearrangements. The ports aren't fragile, but they're not meant to be constantly plugged and unplugged, I'm thinking of the small plastic flaps covering the optical ports.

In short, I would consider investing directly in a UFX III. Sell your existing 802, put the up to ~€1500 into it, and finance the rest with a small loan if necessary, or just wait and save until you have the money.

With the additional 64 MADI channels (at single speed), you'll not only have enough port capacity, but you can also use unused MADI channels for special Submixes, which you can also record via loopback if needed.

With a fully loaded recording interface, it's easy for all channels to be in use, and you might not be able to do something like that.

The quality of the analog section of UFX III (also in the new 802 FS and later UFX II) is superb. It uses the same AKM converters like the older ADI-2 Pro FS reference converter.

I wouldn't worry about the ADI-2 DAC or Pro too much, the delta is small, it is more about features.
But you can at any time later connect an ADI-2 DAC or Pro or even ADI-2/4 Pro SE if you want those additional features.

Now with the UFX III (also in the new 802 FS and later UFX II) you get additionally ROOM EQ and cross-feed and optical SPDIF for both ADAT outputs.

MADI also gives you other advantages … Everything in one cable and the OM4 (or OM3) optical fiber cable can be up to 2 km long between (each!) of the devices which you can connect "serial", one device after the other, which results in a ring structure. For normal use cases between monitoring and recording room, the usual multimode "patch cables" are sufficient, you get them in shops, usually up to 50m long.
With ADAT, you can't chain devices and according to the specifications, ADAT allows only 10m, with RME up to ~15m.

If you're wondering why I consider relying solely on ADAT expansion to be too limited in the long run...
If you can't afford RME converters as an expansion (e.g., M-1610 Pro), then I would recommend a Ferrofish Pulse 16 as the next best option.
The Pulse 16 has 16 analog I/O, so you can connect up to 8 external stereo devices (EQ, compressor and what not).
With that, both ADAT ports are fully utilized at single speed.
It would leave no room to connect a mic preamp unless you sacrifice some analog port capacity or opt for external preamps, which are not only expensive but also take up additional space.

Additionally, at double speed, two ADAT ports won't suffice.

Therefore, MADI provides a better and more future-proof foundation if you intend to connect more devices, whether converters or preamps. Assumed the number of devices might pile up in the future and assumed that you might expand both, number of Mic inputs and analog line inputs for connecting external devices.

True, but what about future demands, I mean this comment, I quote it just in case you overlooked it:

"I am ready to spend €1.000 - €1.500 (roughly) for a start, but of course I try to get the best option for the least amount of money. I am more than happy to upgrade my setup over time in the future, hence I am looking for a starting point which is as future proof as possible. And yes, like every studio owner, you at least consider feeding in analog outboard gear in the future (analog summing, external fx like compressors etc.). It‘s not the main priority, but I won‘t exclude this option in the future."

How do you add an external EQ, compressor or channel strip on the BBF Pro?
Not so easy and straightforward. This makes the BBF Pro to an unsafe investment.

In contrast to that the UCX II is in budget and even the 802 FS if he saves some money.
Then he needs to think about whether UCX II is preferred or the 802 FS feature- or connection wise.

Hey, he is not buying with the intention to sell later
He is buying according to current and future demands, and he stated that he is ready to spend of up to €1500.

Then it makes no sense to think about a recording interface which has not enough analog I/O to connect external devices (one of his future requirements). Besides this, the BBF Pro is designed for mobile use and has a few limitations to be able to run with USB2 bus power.

Regarding selling. He is safe with any RME product, up to the flagship units. Every RME recording interface sells without any issues, usually around 75% of street price.
Complete accessory and mint condition is a plus. This is why I am keeping the original package, keeping everything in mint condition to be able to get a good price and the interest of buyers.

So if BBF Pro FS would only be a temp. purchase this would mean that he looses approximately 25% when selling which is currently €819*.25= €204,75. This money is IMHO better invested into an interface which holds for at least 5+ years, with the BBF Pro its foreseeable, that it won't last long. He already had an 802.