Best components to optimize Martin Logan 50XT and Edge in walls

"I think ethos is on the right track as I was thinking Denon/Marantz with pre-outs and a good power amp immediately."

Why would you buy an AVR receiver when you are not going to use the power amp section? Use that same budget on an A/V preamp/processor to use with the power amp. As far as matching your rear speakers to the front, you should read this article.

https://www.soundandvision.com/content/matching-front-and-surround-speakers

I agree with auxinput that its possible that you just don't love a warm sounding AVR/amp, but its also possible that you'll enjoy a higher quality and more powerful one just fine...you'll only know for sure once you've tried it. Look for somewhere that will let you audition the equipment or at least has a good return policy. The Anthem MRX720 (~$2500) is great and will probably be my next AVR. I haven't heard the Cambridge (~$2000) myself but the reviews are fantastic and it is largely what has kept me from buying the Anthem already. The Cambridge only has pre-outs for front R&L channels, doesn't have HDR nor Atmos/DTS:X compatibility, and doesn't have room correction built in...none of which may be important to you at all, but are things to be aware of. They both probably sound better than what I suggested but also cost more so it will be a matter of balancing what matters most for you and your budget. If you spend less on the surround speakers or subwoofer than I outlined above, you can definitely make either work within your budget and they'd be great.

Once you've cut holes in the wall and installed speakers, convincing your wife that you want bigger holes / bigger speakers might not be an easy sell...that being said, big holes for big surround speakers might not be a battle worth fighting at all when it is an area where there is definitely plenty of room to save money vs the ML Axis I mentioned above. I just went with something close to your Edge speakers and tried to make the rest of the budget work as a starting point to consider...I have no personal experience with ML's in-wall speakers so can't tell you much other than keeping a similar profile with what you have is at worst not a bad idea.

Personally, I'd suggest not starting off with both a new receiver AND power amp. You're right that a power amp is more useful long term since the pre/pro is what tends to change more quickly, but if you get a really good AVR, it may simply not be necessary. You're also likely better off getting a good quality AVR than cheap separates. Good separate pre/pro & power amp (i.e. Emotiva XMC-1 @$2500 & XPA-5 @$1600 for a reasonably priced example) are going to be tight with your budget, especially since you have other needs and would start having to buy interconnect cables. A good AVR is less expensive and you can always add an amp for any channels that have pre-out connections down the road while still using the AVR as a pre/pro and as an amp for some of the channels or zone2 functions... later replacing the AVR completely with a better pre/pro if you feel the need. If you're interested in used options, that makes separates more feasible, but not sure about your feelings on buying used.