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Although I could add files to mirror.ppa.trinitydesktop.org in the same way, they would be removed at the next regular update from the François source :)
Understood!
The trinity-tdm package problem seems to be related to SElinux: https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.r...
Good clue! Did the usual sealert/ausearch/audit2allow/semodule (after telling yum to grab the rpm for me and running the -Uvvh ...) and now all appears well.
Wish me luck that Nvidia doesn't kill me on the reboot! :-D
THANK YOU, Slavek!