What plugin??
If you are talking about AMD Radeon ProRender's plugins, there is not any support for Daz Studio, at least not yet:
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If you are referring to something else you can share your knowledge
I have looked further into it, and I admit I have made a mistake. You are correct. Saw a plugin, but had fine print won't work with AMD. You are correct. However, I think it could be done eventually with a software plugin. Eventually, something will come out allowing the use of AMD for this, but you are right at this point in time Daz & NVIDIA got a little something going on blocking out cheaper alternatives such as AMD for this particular task. AMD is a bit more open source, eventually, something will be there.
In theory, it should work. Problem needs to find a correct driver and that may not (and likely does not) exist yet. What I said before is true, and would make a better machine setup, but wouldn't be able to do the job for this particular task so the point is mute.
Upon further analysis, I would be willing to bet that Nvidia and daz3d are connected in some way on a corporate level. The ONLY reason Daz would have to not allow more compatibility is to sell more Nvidia cards. So yep, you are right. In theory though if it were compatible the setup I described would work better, but you pointed out it won't. So would go with some of the cheaper Nvidia cards that work as an alternative. Packing all sorts of video cards on a single motherboard is still a bad idea, and it just makes more sense
4 good machines > 1 great machine