I deactivated the compat plugin first, forcing the error immediately (btw. in error.php ). Found the respective line in VSC. Correct and get next error.
Later I systematically went through all the to-be-replaced classes: search in VSC and correct one by one.

I've been in a situation similar to yours:
My proposal: Do it yourself!

The problem is that i have no idea which files or what to do or what to change

Perhaps Ciaran will provide us with instructions on this

The problem is that i have no idea which files or what to do or what to change

What to do can be found here: https://manual.joomla.org/migrations/44-50/compat-plugin/#class-aliases
You have to go through all the PHP files of the template. Best use VSC.

Hello dear J51 community,

I also had it rewritten today using the Carmen template with the help of AI, and it now runs under Joomla 6 without the Behaviour – Backward Compatibility plugin. I'm also no longer getting any errors.

Dautrich, thank you very much for your support and advice.

(Working time approx. 1.5 hours)

All the best for 2026 and good health!