As a person with over 5,000 cassettes still in the house and owner/operator of several Nakamichi Cassette decks I have been reading this thread and waiting for the right moment to chime in.
1] You are NOT crazy, IF this is what you wish to do, then go for it, obviously you are aware of the limitations.
many people still use the Sony D6 or D5 to do recordings, just not many doing the types of live recordings most on TS do. Check out
tapeheads.net if you haven't.
2] How about trying dynamic microphones? No need for a pre-amp this would use the D6's Sony pre-amps (not ideal, but cuts out a piece of gear). Of course, you could still use a pre-amp if you choose, but would not be restricted to having one.
In my sig line I still list my Beyer M201c's that are at least 30 years old. I stealthed with these many times, they are a bit larger than the Line Audios. There are Sennheiser dynamics and others out there.
3] As far as blank tapes, after years of listening back to what we recorded in the 1980's and 1990s (I stopped analog late, as in 1997) I can say the best retention of the analog signal seems to be type II tapes. We went to type IV relatively early in the game and I must have 6-700 metal masters along with thousands of chrome masters, and ALL OTHER THINGS BEING EQUAL (playback decks, cleanliness, electronics, etc) I believe I prefer the chrome tapes and can actually point to where we used both Chrome and Metal in different decks but plugged out of the same mics that the chromes have held up better over the years, less playback issues and more high frequency retention than the metals. I spent hours on tapeheads reading and querying the folks on there who KNOW their cassettes and can figure that 1] the metal oxide coatings are "rougher" on tape heads and 2] seem to not have as much signal retention after 25+ years and 3] the chrome coatings of the higher quality brands such as Maxell and TDK seem to have been more "consistent" which, of course, leads to 4] I have had some of the best, but certainly not ideal storage conditions for my tapes. never stored them flat, always on their spines and mainly in AC or temperature controlled environments.
all that said, the types of blanks available nowadays are either old NOS or the newer manufacturers which I have not tried. There are sellers of claimed NOS and used tapes on tapeheads