I've got about [5 or 6] different boxes (sticks, various iterations of the Google/Android/ONN box, etc.), tried [4] different ISPs (one is 1G fiber // AT&T, one is Spectrum "fiber powered" coax for the 'curb to residence' and one is a rural power co-op 300M fiber, Brightspeed DSL - have used probably a dozen different apps - TiViMate, Sparkle, Smarters PRO, etc. - and still got regular stutters / with playback varying from hiccup and then replaying the last 3 seconds and go again - to freeze up - to even timing out sometimes (this is on LIVE TV, every few minutes at times like prime-time and several times an hour at other times during off-peak times). VOD was always mostly OK, occasional buffering - but that comes with the territory on IPTV. Tried WITH and WITHOUT VPN, different VPN providers (SurfShark, Nord, Proton), close servers, foreign servers - low load servers. Bandwidth, latency and jitter were all testing good on all ISPs. I tried a couple of different servers / playlists, granted only one was a 'paid' service but they all did the same / similar periodic stuttering. Restarting / force close app, clear cache - tried it all. Some things helped for a few minutes, but not very long.
Not really "buffering", just like they missed a tooth on the gear and had to back up and regroup, then they would usually skip a few seconds later on to "catch back up to real time" (again, Live TV).
Guess what seems to have fixed it? Doesn't make sense from one angle, but totally makes sense from another.
I changed the Protocol on the VPN away from Auto/Wireguard and started using OpenVPN/TCP. The difference is staggeringly better. Streaming doesn't actually need THAT much BW, but it needs to be clean and timely and reliable.
Of course, with OpenVPN you get less throughput when testing speeds and can cause more processing overhead on (router or box, depending on where the VPN is set up) - because of the overhead of OpenVPN vs Wireguard - (Analiti tests, as well as others show slower total throughput and higher latency on OpenVPN vs WireGuard) - but the reliability appears to be the built-in error correction of TCP of OpenVPN instead of the UDP 'send and forget' of Wireguard. Turns out what I thought all along - not a bandwidth issue (on my end at least, could be on the server(s) end(s) I suppose).
Your mileage may vary. It came down to ONE thing that a guy I saw on a video nudged me into trying that... and it worked. For me.