@Rowanor
Can you attach a dxdiag per this link
Ensure exceptions exist for launcher.exe , brwc_swtor.exe and swtor.exe in the antivirus and firewall exception lists.
Worth re-applying this particular directx redist -Link
Given what you have tried already have you considered switching to non streaming per this link this removes bitraider processes whilst troubleshooting and allows the system to be taken to a full clean boot and able to play without issue.
Edit- also double check windows power option is set to high performance (dxdiag should show this in anycase)