I try to avoid trying to do everything on one guitar in one tuning. Experience has shown that it never improves over the original. A baroque lute does baroque lute music better than any other instrument. Ditto a baroque guitar, a harpsichord, a cello, etc. One of the reasons I took up the lute was to avoid all the questions and problems you outline above. But...it gets expensive! Instruments are not cheap, as you know, so if you simply MUST play music not written for your instrument and tuning, compromise is essential.
I haven't tried playing G tuning repertoire in standard tuning (if I can call it that - Russians see G tuning as standard, of course). And I certainly haven't tried playing Spanish music in G tuning. That's just crazy!
No one tuning is better than another in itself, but G tuning is better for Sychra, just as standard tuning is best for Tárrega. Actually, I equate Sychra with Sor, and Sarenko with Tárrega, but that doesn't bear much analysis, as they were all quite unique.
The eight-string Spanish-style guitar can sound great in Russian tuning, plus one extra string. Here's our Mårten doing just that: