1) You're misusing your terminology. Multithreading is when an application uses more than one processor threads. HyperThreading/SMT is when a processor can run 2, 3, or even 4 threads, seen as logical cores by the OS, per physical core. The benefits of HT/SMT vary depending on the load on the processor core, as it depends on the CPU core not being fully utilized. TechPowerUp did an article on the 3900X in creator type tasks (encoding, rendering, and other non gaming tasks), and found the overall difference to be 10.5%, but depending on the task it could be only a difference of milliseconds. I linked the full article and the graph except below.
https://www.techpowerup.com/review/amd-ryzen-9-3900x-smt-off-vs-intel-9900k/3.html
2) Ryzen 3000 series (Zen 2) performs about 15% faster per clock than Ryzen 2000 series (Zen+) due to architectural improvements. Personally I would go with the 3500, but the 3500 hasn't officially been released yet.