upgrade, and NEVER run RC in production.
They are for testing only.
You should be on 7.8.x No one will help you with EOL code.
Nope.
Because of limitations of node anything above 200 users can be problematic.
For 20k you really need EE, and that includes microservices. That is not free.
I also note last time you asked a question you found a solution but never posted it:
This is comminuty support and that includes both taking, and giving back.