The "Future HDMI 2.1 upgradability" is going to kill you. There are only a few processors that would fall into this range:
Bryston (maybe, but it is a very small maybe)
The NAD is the cheapest up-gradable processor in this list. However, I don't know if you realize that HDMI 2.1 doesn't really give you much. The main new features are only support for high frame rate 4K (120hz) and 8K video. I am of the opinion that 4K is already overkill and 8K really doesn't give you much. I actually still run a 1080p projector filling a 105" diagonal screen. While I can easily see that 4K would be an improvement, 8K is just not needed (but it's the next marketing item for mass market consumer electronics).
There may be a small number of processors that support HDMI 2.1 already. However, pretty much all new processors support HDMI 2.0 and HDCP 2.2 (which is what you really want anyways).
If you really want HDMI 2.1, you will likely wait at least 4-5 years for some of the higher end brands to support that. 8K is very new and I wonder if it will actually end up a dead-end (like 3D video did). It really doesn't add to the videophile results. It is mainly used as a filming and editing format for Hollywood where extreme resolution is important when you are doing complex editing and graphic effects. Once it's mastered, the down-convert to 4K or 1080p will be just fine.