I have two FTV 4K Max Gen 1 set up to block automatic updates on FireOS 7.6.9.7. Both have OTG connections with external USB flash partitioned for internal and external storage. Some apps are run from USB so I use up only 3.3 of 5.33GB internal space. Most often used sideloaded apps are Projectivy, Launcher Manager, Stremio, Sportsfire, and SmartTube.
Everything is working. I remotely connect for ADB. For major streaming subs (Prime Video, Apple TV+, etc) my LG TV’s WebOS has native apps.
Should I replace the FTV sticks with the 4K Plus? I can foresee that some app updates may no longer be compatible with an older FireOS, but I’m not sure if this is compelling enough to fix things that have not broken. Yet.