I had the same experience, trying to self-host my VPN or other evasion solutions only served to get my server and domains banned for all other purposes from inside the GFW. The symptoms were exactly the same as you described (at first, they "sort of work, but unreliably, with extremely minimal bandwidth and would suddenly stop working after some time").

In the end, I'm not going to try that anymore, I haven't been in China for 2+ years now due to COVID, but next time, I'll hope my server is out of the blacklist again and hope I can access my (self-hosted) emails and other normal services that don't try to evade it.

The student VPN of another Asian university or the employee VPN of a well established company seemed to work last time. Not sure if that can be counted on reliably though...