Hi Simon
I don’t use Podium (VRay and Twinmotion) but it’s basically the same question.
My only recommendation in this case would be to get a GPU with the maximum VRAM (so probably 8 in the iMacs case)
I posted some stuff a couple days ago in response to a similar question (not exactly the same but some of the concepts are transposable), it might help…
Podium for MAC is not GPU accelerated , for many of the same reasons as detailed in that post.
It is worth considering an eGPU should the situation change for the better in the future.
Hope this helps.
Cheers
Rob