Please be aware that factory resetting a Roku device should be a last resort when troubleshooting a channel playback issue. In order to insure a channel clean install, we would recommend removing the channel, restarting the Roku device, then adding the channel back again, specifically in that order.
Removing a channel, restarting, then re-adding the channel has also been attempted with no better result.
Did the Youtube channel stop working for specific models of the Roku for some reason? It's the 2018 Ultra version which is a bit old. I don't see why it would, but that's the only explanation I can find at this point, especially after a factory reset and it still doesn't work.
If the problem is similar to the one I think I am having I would try a factory reset, but then re-setup the device using a new email address instead of logging in with your existing Roku account. That way it wont install any saved data associated with your original email when the Youtube channel is re-installed.
We appreciate your patience regarding this matter.
For the issue you're experiencing with the YouTube channel, we recommend contacting their support team directly for more information and better insight into how their channel works on Roku. They'll be the best ones to help you out with that matter.