Unless you or your partner are among the tiny minority of folks with Golden Ears,* I suspect that changing your A/D converters etc is not going to make a big enough change to justify spending the money.
* Oops I just repeated myself!
Since you have already treated your room, maybe you should look at your monitoring system?
Obviously several things affect the end result that you hear but If you really are determined to change your actual interface, I have to agree with you on choosing RME. My 2 systems are rock solid after many years, despite buying BOTH of them used.
Over the years, the things that have made audibly significant changes for me have been room treatment, buying my Rode Classic I valve mic, buying the RME interfaces & buying a pair of Unity Audio The Rock II monitors.
Marginal improvements have come from a reflexion filter & changing from Behringer ADA8000 to Focusrite Octopre, but these combined with an old JoeMeek Studio Channel and a GAP 73 have given me everything I have needed in terms of clean - to - character preamps without spending an arm and a leg.
Coincidentally I have been recording in a pro studio recently & was interested to see that the few outboard pres they had were all midrange to high end focusrite. Mostly they use the pres in their desk. I also made me realise that my stuff doesnt sound as good as theirs primarily because they have LARGE well treated rooms. Most of them bigger than the whole ground floor of my house. *sigh*
Another thought: When you say you have a good range of mics, what exactly are we talking about here?
I have a bunch of the usual Shure & Sennheiser dynamics, plus 3 different stereo pairs of SDCs & my Rode Classic, a lightly modded Oktava 319 & a couple of generic LDCs. 99% of vocals (mine) I use the Classic alone, or the 319 with the GAP 73, although if I want a more 50`s 60`s sound I use the JoeMeek. Excellent character preamp & a classic compressor sound.