Usman Asar

if you're comfortable with MySQL, then I can assure you, nothing will happen, but you got yourself a better product, Maria DB is enhanced with enterprise tools and services, Optimized for performance and offering high availability, security and interoperability beyond MySQL and other databases.

the 10.2.32 is least that 3.9 requires and that's 10.2 that came with default with Barracuda
Just FYI, MariaDB is created by same guy who developed MySQL (Monty), once it was sold to Oracle, he, on the fear they may not commercialize it, started a new fork of MySQL, MariDB. So everything is same in both making it a drop-in replacement.
you can use 10.5.4 as well, no issues there
but be sure, its 64-bit version

if I were you, I would have used Microsoft SQL Server, even people on Linux were considering it over MySQL, as database gets heavier, SQL Server is 3 times faster than MySQL/MariaDB

Yes, just read through, MySQL 8 is indeed on Barracuda default

MariaDB is drop-in replacement of MySQL, so you wont be doing any conversion, just mention driver used and you can switch between MySQL and MariaDB in a second.

Workbench will work on MariaDB too, in fact anything that works on MySQL, will work on MariaDB

All those commands that you use on MySQL will run on MariaDB - absolutely no difference.

Windows Server works absolutely fine, I myself have many implementations of IIS and Moodle and they work absolutely fine.