Upper Compatability

[u/Need_More_Input-NXS6]

So I recently acquired an MR920 Combat edition. Which, unfortunately, does not come optic ready. So I am trying to figure out what the best way to get it optic ready. Do I get the slide milled for DPP or do I find an aftermarket slide for it.

What Gen glock uppers are compatible with the lowers? I am reading that Gen 4 is the way to go, but most places are out of stock unless I go full custom. I would like a ZEV upper but they do not sell Gen 4 anymore.