That’s a pretty good box you have and the up/down speeds are plenty. I would suspect the provider of the stream might be the issue or if behind a vpn that can cause buffer as well. If on a vpn try different locations(closest isn’t always better). Another thing that can cause buffer is the dns server in use. As mentioned above Streamio is not really an iptv provider and mostly used for vod.
Premiuize and Real Debrid I think with Stremio its important to keep scrape sizes small say max 300mb. Unlike Kodi Addons e.g. Umbrella and Seren the scraping appears to be better than Stremio
This depends a lot on many factors. I have a killer device and don’t concern myself with Source sizes at all. Any addons having the option to limit sizes I set to 100GBs. Yes that’s one hundred. This allows for a lot more info to be imbedded in the source and I do look for H265 and hopefully a minimum 2160 HDR. This is why it’s getting to the point where the minimum RAM needed is 4GBs. My device is 32GB DDR5 . So I very rarely have any issues playing a vid in Stremio. However for live TV I always use TiVimate.