i love MS SQL, the dialect is very easy to understand and the license is also fairly easy to get.
You want non limited free non commercial? use developer edition
you want limited free commercial? use express edition
you want unlimited non free commercial? anything else.
you will be okey with ms sql express for hundreds (thousands maybe?) of customers.
with tools like SSMS or Azure Data Studio you will have an integrated IDE that will help you do backups and specifics of SQL SERVER at hand,
And! SQL Server is now Linux native, that means that you can install it everywhere.