Well o-ron, I doubt we'll ever get down to a consensus here, but what the hey, that's part of the fun.
I don't know why you "don't want it to sound like a solo piano and violin just happen to be performing in a giant concert hall", if that is really what it is, but maybe that's where we simply differ. The acoustics of a good hall are like precious gold when you have them and I would hesistate to substitute reverb effects, if there weren't some other compelling reason (like noisy ventilators or light dimmers or some other problems in the hall).
Re the instrumentation mics, see this thread:
https://homerecording.com/bbs/showthread.php?threadid=36354
particularly in regards to comments regarding self-noise and output
I record a lot of chamber music, and find that the self-noise is mercilessly revealed in a very good quiet hall, especially during rests, and silences at the beginnings and endings of movements. This is much less of a problem recording contemporary music, which is what mostly goes on here, I suspect. Of course, this gets worse when you mic from farther away, as is typically the case with violin and when you use only two mics to pick up everything including the hall.
A ribbon mic is fabulous for getting the most flattering sound from most violins and other bowed strings, and can be used more up close, but you need a good preamp. A used
Beyer m260 is not prohibitively expensive, typically ~200 on ebay, but they come up relatively infrequently, and being on the fragile side, getting a good one is a little bit of a crapshoot.
I prefer the 603 over the 2003 because I think it has a more even and warmer sound, which I think would be better especially on a good large grand piano, which can generate surprisingly deep and strong low fundamental tones. I would take Marshall's response curves with a HUGE BOULDER of salt (in other words I don't personally find them useful at all as a guide to how the mics sound), but it is rather by listening that you will get tuned in to the unique acoustic signature of each mic. I guess others do find the curves to be more useful, but my ears don't seem to be tuned that way.
Also, the accuracy of these remarkably smooth looking advertised curves is highly questionable -see this link for more info:
https://homerecording.com/bbs//showthread.php?s=&threadid=27030&perpage=40&pagenumber=8.
BTW, my Oktava MC012 has a substantially lower self noise than the 603. I also think it a little smoother in the highest octave.
I like the idea of multitracking in that you basically have two main stereo mics, then use close-up mics and ambient mics to tweak the final mix to taste. Easy to eat up half a dozen channels that way real quick!
Sounds like you have some challenging and rewarding projects ahead, enjoy!