The current(2019) i9 iMac is great, with 32 GB and NVME SSD.
I installed a couple for clients, and decided to built a Hack lookalike because they’re that good, Coffeelake + Intel i9.
I had an 6700k/Skylake before, quite a difference. The i9 is much more powerful.
I don’t think the new iMac series will be much better, Intel hasn’t made that much progress.