You socket names do not match. Even that the symlink seems to be in place, please try to use the actual “modern” socket location in Redis and Nextcloud: /run/redis/redis-server.sock
If that doesn’t solve it, I remember even with 770 mode and www-data in redis group for some unexplained reason during early Debian Bullseye times PHP/Nextcloud was not able to connect. Meanwhile it does in my case but temporarily I needed to set: unixsocketperm 777