As mentioned by muru, running squid with squid -N -d 1 reveals that
2021/07/27 03:20:12| FATAL: failed to open /run/squid.pid: (13) Permission denied
exception location: File.cc(190) open
The solution is to place the pidfile in another directory, or to avoid creating it, as per http://www.squid-cache.org/Doc/config/pid_filename/ place this in the config:
pid_filename none