Update - Success!
When I started my version of MariaDB was
mariadb-admin Ver 9.1 Distrib 10.6.12-MariaDB, for debian-linux-gnu on aarch64
I upgraded in two steps. My first step brought me to 10.7.8
mariadb-admin Ver 9.1 Distrib 10.7.8-MariaDB, for debian-linux-gnu on aarch64
However, when I ran mysql_upgrade it showed this as the first line
Major version upgrade detected from 10.3.22-MariaDB to 10.7.8-MariaDB. Check required!
So I have a feeling that while I might have been running 10.6.12 the version of the database format/structure (don't know the appropriate terminology here) was based on 10.3.22.
I didn't bother to test this LibreELEC because it did warn me that it was a short term support release.
At this point I figured to throw caution to the wind and go straight to 10.11.2 which I see now is a developmental release but it Beta 2 is now working just fine. I'll be making notes on this one so that I can upgrade to the stable version of 11 when it comes out. I don't blame them but the documentation for all this assumes you are more advanced at all this.
Andrew