Problem solved
To use php-redis from Sury repositories:
apt-get install php-redis -t stretch
Then
phpenmod redis
systemctl restart php7.3-fpm
And set the permissiones in redis.sock to 770
chmod 770 /var/run/redis/redis.sock
Now redis is working with nextcloud and can check it using redis-cli
redis-cli -s /var/run/redis/redis.sock monitor