After fiddling around I think I fixed the problem. I edited the MariaDB server config file (located in /etc/my.cnf.d/server.cnf) and added those 2 lines:
innodb_log_file_size=512M
innodb_strict_mode=0
This fixed the “Row size too large” error.
Now in order to get the upgrade script to run and resume the second stage I had to comment out the lines where it tries to copy files over (line 819 to 847). However once the upgrade script finished some Mautic v3 files were missing.
So I downloaded the 3.0.1 Mautic ZIP file https://github.com/mautic/mautic/releases/download/3.0.1/3.0.1.zip and extracted its content in my new Mautic 3 base directory.
I got a final bug where my browser was telling me I wasn’t authorized to access my Mautic web site. I fixed this by restoring the .htaccess file from my old Mautic 2 installation in the Mautic 3 base directory. (and then rewrite rules are working and I have Mautic 3 working fine)
Good luck everybody, I hope this helps someone hitting the same problems.