You are correct- it does not have HDMI and you'd have to have that if you are going with the 800X M2. Though it gets harder to find AVR's with both HDMI and pre-amp outputs.
Yes- you'd need a Dolby Atmos preamp-processor or AVR that is not only MCH but also Atmos capable. And you'd need 7 channels of amplification and at least 2 overhead speakers to make Atmos worthwhile. 5.1.2 is the minimum Atmos configuration: FL, FR, C, SR, SL (the 5) and 2 overheads (the .2). I guess a sub (the .1) is optional, though I'd imagine it's highly recommended since at least the overheads would not be full range. Someone with more Atmos knowledge can chime in on that last point.
Unless you have 7 channels of amplification, I'd imagine it best to find an Atmos capable AVR with pre-amp outputs to use your current amps and then use the AVR's amps for overheads. But I think you have to go pretty high-end to find an amp current enough to have Atmos that also has preamp outputs.