a/v processor

The "Future HDMI 2.1 upgradability" is going to kill you.  There are only a few processors that would fall into this range:

NAD M17
Theta Casablanca
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. 

The type of DAC chip itself has very little to do with the sound (maybe about 5% impact).  In my opinion, the important sections are power supply architecture/parts (about 55%) and the I/V and analog stages (about 35%).

Others have said Audiocontrol has good stereo sound.  It is very hefty at 27 lbs, which indicates a good size linear power supply and supporting circuitry.  However, I don't know if anyone here has direct experience.

I know that McIntosh processors (including your MX122) have very excellent power supply sections.  I think moving to a different processor will really only give you a slightly different sonic signature.  The more expensive MX160 or MX170 processors will likely improve on the sonic signature of your MX122 and will contain the Lyngdorf digital section (which is better) instead of the Marantz digital section.

The Datasat has massive amounts of ASIC DSP chips (4 big ones) for a large amount of DSP processing.  It does have good power supplies, but it use a 25-pin computer plug for all audio outputs.  This means you have the use the 25-pin to XLR break-out cable for audio connections.  I did read one review where the guy said there was no significant improvement over his current Meridian 861.