Yup to clarify I bought two packs of TeamGroup Dark Pro Red modules, found here:
https://www.teamgroupinc.com/en/product/dark-pro-ddr4
SKU is TDPRD416G30.
So, 4 of these, but two are based on Samsung B-Die and two are SK Hynix 4GB A-die, or AFR I think. The B-die is single rank, but the Hynix is not, which I'd overlooked.
The X570 Unify is no slouch, the one I have at home runs my 4x 4000MHz CL19 at 3800MHz CL16 with Gear Down Mode completely stable (in my profile system) so I won't lie that I'm disappointed if there's really no way I can encourage it to at least hit XMP speed with these.
Seems obvious now that the mismatch is the cause, but I don't know if removing the AFR and running higher speed 16GB or keeping all and running low speed 32GB is the right move from here.
I had tried running only two modules before and faced one crash, then assumed it hadn't helped, so that was probably too hasty.