Bitcoin Core v0.21.1  has no problem for solo miner. mining cmd: getblocktemplate and submitblock got no changed and work well.

cgminer 4.11.1 need to fix it to work on gbt_solo mode. The cgminer got three work modes: has_stratum, has_gbt and gbt_solo.
I found it is very hard to run c code when segment faults often pump out. You have to work out yourself as solo users.