Not helping you with selinux, but in general i run to the same problem with ubuntu 20 server without selinux.
I finally find out that you should change /etc/redis/redis.conf
unixsocketperm 700 to
unixsocketperm 770
so that www-data group user can access redis-server.sock , in documentation that is not mentioned.
https://docs.nextcloud.com/server/21/admin_manual/configuration_server/caching_configuration.html