Nice to see you putting in the effort to play detective and by elimination sort out an issue. I think @Crystalblue6161 was spot on with a warning about low storage space. Personally, I've never been able to come up with a "danger number". More is going to be better. Certainly getting up over 2 GB is great, but I can say that .5 GB isn't necessarily the killer number either, but you don't want to be there. I like to keep to two or better. but I doubt 1.8 was an issue. I recently through my own stupidity did a factory reset and TBH it wasn't the end of the world.Update
Week ago I described my problem with buffering, to summarize all the channels were acting up, I have firestick 4 k,xciptv app, at&t fiber and I use nord vpn. I tried all the usual troubleshooting tricks - no fix.
Then Crystalblue suggested low space on the stick. I looked it up- I had 1.3 gb free space. Threw out some apps, clear the cache from the rest- got it up to 1.8 free. Tried to run the tv, still the same problems. I figured I got 2 steps left to try, update firmware and factory reset. Did the first one, my available space went from 1.8 to 2.3 and all the buffering problems went away
Not saying that this is universal fix but it definitely worked for me!