Probably the Tor Browser developers changed something.
The question to ask as per https://www.whonix.org/wiki/Free_Support_Principle would be kinda like:
How to pre-configure Tor Browser using a configuration file before Tor Browser’s first start?
How to set the Tor Browser security slider to maximum using a configuration file before Tor Browser’s first start?
I don’t know if upstream, the developers of Tor Browser are considering this the user.js / set security slider to maximum using a settings file use case. user.js is probably a feature by upstream’s upstream, which is Mozilla Firefox which hasn’t been considered / tested to not being broken by the Tor Browser developers.
It used to work but /usr/bin/torbrowser cannot do any black magic. It can only automate which at least in principle can also be done manually. If it cannot be done manually, then I cannot help to make it more comfortable / automated.
Maybe policies.json (which is also a Firefox feature) (which was mentioned here Tor Browser Customization using user.js (for example for i2pbrowser)) could be used as a replacement for user.js.