I personally wouldn't go that far tbh, but I haven't rly looked into it that much so idk.
To me it seems like they are still maintaining Firefox, Gecko, and Thunderbird (which is more independent tho IIRC) quite well.
I use Thunderbird directly and Firefox through Librewolf
(haven't rly had the drive to look into Thunderbird analogues to Librewolf. Should rly do that tho)
Doesn't seem like it, but users who don't know how to toggle a setting should be kept faaaar away from sideloading