Dx12 is a hard requirement for features like Nanite to work properly, this is set by the engine itself, so there's nothing I can do in terms of supporting Dx11 without sacrificing those features unfortunately. At the very least I would have to cut Nanite completely out of the game, which is possible but as I'm using it for things like foliage I would also need to ensure everything currently using nanite has proper LODs. So it's just lots of extra work
Many of the crashes and problems for AMD card users were fixed in Unreal 5.2, which I have not upgraded the game to use yet. This is because I'm using a custom branch that allows for Dual Quaternion skinning. More info here:
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This custom branch has not yet been upgraded to UE 5.2, but I've spoken with the person who maintains the branch about possibly updating it for 5.2 and they were receptive to that idea, but I don't have a timeframe for when that will happen
tl;dr Unreal 5.1 had big problems with AMD cards, this was fixed in 5.2, I'll upgrade to 5.2 as soon as the custom branch I'm using is updated but it hasn't been released yet