Collation conversion

Only difference between DYNAMIC and COMPRESSED is that COMPRESSED further compresses the data. COMPRESSED is slower because it has to compress and uncompress data on access so the CPU demand is higher, but the storage demand is lower. I recommend just using DYNAMIC.

My guess is large indexes are not working for you because it's empty. It's also possible Akeeba did a pre-check before deciding to change the collation and using math determined it wouldn't fit based off the default innodb index size limit otherwise if large indexes is configured correctly it would've never errored and converted the collation perfectly fine.

For a new website using MariaDB with innoDB engine and utf8mb4 it seems impossible to install CB.
We encounter an index > 767 error.

Trying to find a solution it seems like according to this you shoul set some globals before executing script.
stackoverflow.com/questions/43379717/how-to-enable-large-index-in-mariadb-10/43403017

SET GLOBAL innodb_file_format=Barracuda;
SET GLOBAL innodb_file_per_table=ON;
SET GLOBAL innodb_large_prefix=1;

However these variables and the script should be included in CB install script. We cannot set these independently of the install

Would it be possible for you to upgrade your install script ??

Joomla nor CB supports MariaDB. You are using an unsupported database. Use MYSQL or you'll need to troubleshoot any database issues your self. Joomla likely will never officially support MariaDB as noted below and so neither will we.

github.com/joomla/joomla-cms/issues/14141

However these variables and the script should be included in CB install script.

No they absolutely should not. Those modify how the database stores. We will never alter a users database in such a way. This would be dangerous to do within our install script.

We cannot set these independently of the install

Yes you most certainly can. They are simply SQL configuration settings. Typically you'd just change them in your "my.ini" file if changing them via query does not work.

Would it be possible for you to upgrade your install script ??

No.