When I found the issue with content manager loading forever I used the addon linker to reduce the number of community addons and for my case the content manager worked again from that point, when I had less than 1028 community items active.
So I used a trick and packed my livery extensions into packages (one package for each aircraft type, using one livery as “start”, renaming it’s top folder into for example Asobo_a320neo_livery_collection, copying just the folders under the SimObjects\Airplanes of each livery for this type into the Airplanes folder of my new collection and updating the layout.json file using the MSFSLayoutGenerator.exe tool). Such way I reduced the numbers of cummunity addons enough to get less than 1028 addons allthough still using all my original addons. And guess what - yes, it worked with the same addons, that did not work before. This means, that the problem depends alone on just the number of addons, not on problems of specific addons.
Here a screenshot how it works to pack liveries together:
Step one:
Step two:
