Hi, my personal opinion / experience:

1. It cannot be diagnosed remotely. You have to try something like that yourself with different devices in listening tests.
Everybody has different ears, listening habits, a different sounding room and if you produce music, then you have additional requirements for a production environment compared to a pure listening environment.

2. Unfortunately you didn't give any concrete information about the devices in your current setup or whether the room has already been acoustically treated/optimized. By this I am getting absolutely no idea whether a €1800 converter could be perhaps beneficial for your setup or to better spend the money elsewhere.

Generally speaking. I would put the money in areas, where the biggest deficits are and where an improvement would most likely make a much more audible difference compared to investing solely in a more expensive converter: speaker/amplifier combination (also suitable for the purpose) and room acoustic.

In the field of AD/DA converters, one tends to deal with minor differences that only really come into their own when the things that have a more significant influence on the sound have been satisfactorily resolved.

On the other hand an ADI-2 Pro FS can give you a lot of fun and value, not only because everything is implemented very cleanly, but also because the feature set is unique, offers many advantages which is simply fun to use every day. I couldn't live without one, in my case even two, one for the recording corner and one in front of a high-end HiFi setup.

Some reports regarding your expectation to get better sound:
a) I got a report from a forum member, that the ADI-2 Pro improved the sound in his home studio compared to using other non-RME recording interfaces.
b) in the converter shootout, see the blog article below, a mastering engineer compared the ADI-2 Pro to other much more expensive high end mastering converter. You can download and compare the recorded soundfiles. The outcome for him, working with different converters, is, that some converter have sonic advantages for the one or other music material and one of the virtues of the ADI-2 Pro is, that he seems to have advantages esp. in the bass area. Watch the video.

I collected many interesting information about ADI-2 DAC and Pro in this blog article, maybe worth a look for you:
https://www.tonstudio-forum.de/blog/Ent … ses-EN-DE/

Another thing: basically, I've found that better components sound better even in acoustically untreated rooms. But only if you don't listen too loud, like I do, then even an untreated living room (besides couch and carpet) doesn't create too many problems for bass. Or in the recording corner where I listen in the near-field, but also at more lower listening levels.

I think you also have headroom in terms of monitors.

I would get active monitors, this ensures to have a combination of amp and speaker that fits and you do not pile up more and more equipment. I would avoid getting a subwoofer based system to make things easier.

I am very satisfied with coaxial systems, e.g. those from Geithain. I own a RL906 since many years.
Perfect for the nearfield and a nice sound stage.
Read some reviews, then you will learn about the company.

Maybe you want / need some bigger. Although Bass presentation of this monitor is already quite nice.
I would listen to many active monitors, also very expensive ones, to get an idea whats possible,
what you can get for which amount of money, before making a selection.

I would not expect much from a better amp! In blind testing amps with pro audio engineers there was no difference. If anything at all they very slightly preferred the very cheap amp, 200$. The others were in the 1000-10000$ range.
In control of bass, damping factor, I would say 98% room, 1.99% speakers, if decent and 0.01% amp.
If getting a adi2 will be audible depends on what you have now. If it is not really old or bad you will probably not hear it in a blind test.

I would say audio qualities like fatiguing, transparency etc are speaker qualities and bass problems are from room for 99%. Sound stage a real mix of speaker and room.
Throwing money at the wrong thing can be a bottomless pit.