Latest available version is Mariadb 10.3.
Check out this post for more details about NethServer using the “old” mariadb.
No need to install/configure NC manually because Nethserver does it for you.
I’d uninstall the remi php versions that were installed with php-scl because they’re not needed anymore and php-scl is not going to be updated and just keep the rh-php* ones.