Listen to what everyone is telling you: the Focusrite has its deficiencies, but sounding "thin" is not one of them. It is measurably transparent and it will not add any character of its own. It will NOT sound better than an RME or a Motu, or even a Merging if you had the cash. The deficiencies of the Focusrite are lack of some of the features you will find on its more expensive competitors. Like a full featured mixer on RME, bulletproof drivers on RME and Motu, and so on.
You have been provided a graph that PROVES that the Focusrite will not add any character, and you dismissed that as "unscientific". The measurements show it won't add any colour, and that should be the end of it. Yet you continue to insist that there is a difference based on random Youtube videos using different recordings and different microphones with unmatched SPL's - the very definition of an uncontrolled test. The only person being unscientific here is YOU!
What you want is someone to make a recommendation of RME sound quality for Focusrite money, according to YOUR perception of RME sound quality. That's not going to happen on ASR. Maybe go try
Gearspace (formerly Gearslutz), there are some pro audio subjectivists there who might be more willing to entertain you.