Hi, I saw a post with similar problems on a higher end system, i.e., great performance on many games but struggling with MSFS. The OP eventually solved the problem and it was related to a setting on his ASUS motherboard (just trial and error to find this setting).
To the OP: maybe read over this and see if your MOBO has this setting and if it helps. The post I linked below from just the past few days had a very strong hardware setup (somewhat similar to yours) and the user was getting like 10-15 FPS in MSFS yet his GPU/CPU were very strong and he had tried many of the things you try above with no luck until he turned off a feature on this MOBO that was related to multi-threading (I think). Good luck and it’s frustrating as hell to spend so much time trouble shooting an expensive system.