Which product do you own? There is no such thing like ADI-2 FS Pro.
ADI-2 FS? ADI-2 Pro? ADI-2 Pro FS? ADI-2 Pro FS R BE?

EDIT: from an older post from you, it is the ADI-2 Pro FS R BE. Could you please add the relevant parts of your setup into your forum signature, then it is easier to answer your questions.

Nevertheless, all these products with analog input do not have an Inst (Hi-Z) input integrated.

You need something like an instrument preamp, bass amp line out or (best active) DI-Box to bring the weak high-impedance signal to line level.

A combination of RME recording interface (with digital mixer and instrument input and ADI-2 Pro would be ideal.

See my blog article how to integrate the ADI-2 Pro FS R BE into your environment.
https://www.tonstudio-forum.de/blog/Ent … our-Setup/

Here an Excel comparison sheet where you can compare the features of different USB based recording interfaces:
https://forum.rme-audio.de/viewtopic.php?id=35156

Yes, this is also a good idea, saves some money, but a D/I Box and (probably also) DAW need to be purchased.

The question is whether later on, requirements could arise that would necessitate the use of a recording interface with more ports and a digital mixer anyway.

Usually, it’s like this: you start small, and then the requirements grow.
It seems he doesn’t have a DAW yet either.

Possibly, a combination of a recording interface and ADI-2 Pro might be more advantageous in the medium/long term.

He could then connect various audio sources, both analog and digital, mix them freely in TotalMix FX, and record via loopback using any program (preferably one that supports ASIO drivers).
I often do this as well because I don’t always feel like starting Cubase just to quickly do a guitar overdub.

In that case, I mix the guitar (played through a stand-alone VSTi) and the backing track together. The guitar also gets effects mixed in from an external FX unit (in parallel). Here, other features of TotalMix FX are also useful. With PEQ, I can cut off low frequencies to avoid clashing with the bass and drums. With fader groups, I can link the guitar signal and the FX intensity so that I always have the right amount of effect, depending on how I adjust the guitar’s volume in the mix.

Further advantages of TotalMix FX: I can create a submix for headphones where the guitar is a bit louder for a more comfortable playing experience, and another submix can be used for loopback recording where the guitar integrates better into the song or backing track. You can also record with other friends and musicians by using Instr and analog inputs, etc. If you should need MIDI for a master keyboard, most recording interfaces also have MIDI I/O.

In my opinion, the combination of recording interface with TotalMix FX and ADI-2 Pro FS is the more flexible solution.
If the Instrument input has no problems with the signal strength of the bass, you wouldn't need a D/I Box.
You can use an inexpensive DAW program like Reaper to perform the loopback recording.
Later you can dive deeper into this and make recordings and mix and master as you like.
The recording interface is then a solid basis for future work / requirements.

Finally it's also a budget question and this has not been communicated / asked yet.