Ok, I have the exact same thing and i will try to help you with what i can.
First, Firestorm can't do anything about that. It's not a server problem, but a blizzard problem. In 8.3 on retail, the game suffered from memory leak and this has been fixed. But in 8.3.7 the problem come back for a lot of people and he was not fixed. In SL, that's seems ok.
Like you say, using DX11 legacy was the temporary solution but it's shitty as f**k. If, in 8.2, you used the DX 11, i recommend you to use DX12 if you can. For some people that's fixed the problem. In my case, i can't use DX12 and for a week i was not able to play the 8.3.7 cause of big freeze. But, after lot of research, i can now play with DX11 and my normal FPS with very rare freezes.
N° 1 : You need to disable the "Talking head box", you can do that with the addon "BeQuiet", or leatrix.
N° 2 : If you use RareScanner (or others addon like that), you need to disable the 3d model in the pop up
N° 3 : You need to disable addon like "Immersion" and "Storyline"
N° 4 : If you use Addon to custom your UI, you need to disable any 3D portrait
N° 5 : You need to not open any panels like Tmogs/mounts etc. If you do and you don't freeze, continue to play, but if you freeze you need to restart the game.
To use your character screen without freeze, you can use the command " /run CharacterModelFrame:Hide(); " . I recommend you to make a macro with that and use when you log in. You can reverse by changing "Hide" in "View".
If you freeze, you need to close the game, wait a few seconds to clean memory, and restart. Victory windows of island expedition and BG can freeze too but it's rare for me.
I know, it's not good.. But, that's let me play and i freeze/crash rarely since all this things.
Hope this can help you, or other people with the same issues.
