Given your stated goal, a 3700 + 2ch L/R amp of your choosing is most cost effective and offers the highest objective fidelity. HOWEVER, it's unlikely these benefits will be audible under real world conditions when compared to any other unit in the AVR lineup.
Additionally, if stereo upmixing is of interest to you I find Auro3D is the only codec worth using and you need a 4700 minimum to get that feature. Dolby with center spread is OK and will also leverage front wide but Neural:X doesn't do it for me at all.
Worth noting since you mention a projector, 7.1.4 for Auro is somewhat incompatible with Atmos which is the far more common format for movies. You can compromise by putting rear heights at a wider angle but like all compromise it probably doesn't get you either of what you want (I've never tried myself). For that reason alone some people go to 8500 granting them enough outputs to wire all speakers and switch on demand. The 6700 can do it, too, but it requires an outboard 2ch amp to support all those connection. Still more cost effective than a 8500, though, given VTV Amplifier's crazy $1k price on Puriif EVAL-1 units for L/R.
I agree 8K is a long way off but I tend to keep things until they literally can't go on any longer so I aim to future proof every purchase. That's a different philosophy for everyone I suppose. I also plan on adding a projector once the 8K UST units hit their stride, probably within 3 years.
As another poster has mentioned, only the 8500/A110 can be configured to fully disable the amps on a per channel basis. With the other units, you can power off the front amps or all amps but nothing in between. Currently I use my 6700 in the "opposite" way most would while I figure out how I want things - onboard amps for Kef R-series LCR and active Neumann for surround - but I doubt I could tell the difference by adding a 3 ch. and switching to full preamp mode. my room is all glass, brick, and hardwood so I have far bigger issues to address before targeting the last ~10dB of SINAD.
Long story short, really focus on features when it comes to these units. Both now and in the future: how many zones, how many speakers, how many amps do you want/need to bring to the table, Auro, DTS:X Pro, etc. And don't forget to factor in room treatment to your budget, if your situation allows. Audyssey's respectable given how easy it is to use but it's not a cure-all and never will be.
I realize my question above is maybe a bit broad, but I'm looking at replacing my Denon 3500 7 ch with a 9 channel AVR. I think for my needs the 3700 would fit the bill, but found a place where the Marantz is very similar in price. 
