As you are apparently a long-term Mac owner, you know that Apple will drop support for new operating systems on older hardware sooner or later. A loaded 2020 iMac would make more sense for a longer usable life in that regard.
As far as TwinMotion - note that you can add an external GPU (eGPU) to the iMac via a Thunderbolt port in the future if you find you need better graphics performance, including multiple GPUs. The extra heat of course will be handled by the external box at that point.
This benchmark, for example, shows very similar graphics performance with the same eGPU in both MacBook Pro 13" and the much more powerful MacBook Pro 16". As noted there, the 13" falls down on pure CPU (ARCHICAD) performance). These tests were done with a single GPU in the eGPU.
https://barefeats.com/macbook-pro-13in-vs-16in-5600m-and-egpu.html
That same site has another set of benchmarks - again of a 16" MacBook Pro which I know you're not looking at ... but the Blender benchmark includes times for an eGPU with one GPU card and another with TWO cards. The test with two cards shows half the time as one card... hence another option for TwinMotion if the iMac alone does not perform well enough for you.
https://barefeats.com/16-inch-macbook-pro-egpu-added.html
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB