Andrea, I appreciate the additional testing and results you provided since it is a key library. Some commentary below.

Test renderings, using x265, work fine, except using the 10-bit and
12-bit presets fail indicating that those pixel formats are not
supported by x265 encoder.
Using the multibit appimage (without the new patch), the 12- and
10-bit renderings work, but the vaapi preset does not.

To get 10-bit and 12-bit you have to apply ./thirdparty/compile_multibit_x265.txt and move the 3 patch files WITH the new names (i.e. instead of 3.5, the 17122023) to subdirectory ./thirdparty/src and then do a build. BUT I have not checked to see if the line numbers are still valid in the patchsets. I will try that late today or else tomorrow.

Probably the vaapi preset does not work because I created the appimage on a laptop with an out of date vaapi.

I wonder if the separation between multibit and non-multibit versions
is still necessary.

well, I thought multilib build was slower, not at rendering but at build itself ? It up to Phyllis to decide ....

I will test this again, but I do some many builds in a single day, it just slows me down a lot and my laptop sometimes overheats too.