What if anything other than max cog is the difference in the XTR M9100 and the M9120 derailleurs?

Background: I'm 64 yrs old and 6'4" tall, even in sight of single digit body fat I'm ~80±kg and just don't have the watts to get up the steeps any more. I'm currently running a 2x10 (XT34/24 rings with a 42 rear cog). The 24/42 combo is good for most stuff but honestly there are some days I'm wishing I had a bailout gear above the 42T.

It's coming time to replace some parts and I'm looking at going 12x, esp since there are now options out there for 12 speed cassettes that will fit an HD freehub. . The idea of 1x is appealing, but in order for me to get the same gear ratio I'm pushing today I'd need to get a 28T front. Granted I'm not fast by any definition, but I think that 28/11 is going to be pretty limiting on the top end in flats and downhills.

In looking at new rear mechs I have noticed that Shimano lists the M9100 as a 1x with a 51T max and the M9210 as a 2x with a 45T max. Now I've been doing this long enough that I remember when it was a 3x so 2x isn't bad to me. That said, my question is can I run the M9100 in a 2x configuration with a 45? Would doing that would reserve the option to change to a 1x in the future and pop on a 51? Or do I need to go with the M9120 if I want to stay with my current 2x configuration (while gaining from 42T to 45T)