As servers for games are shutdown I occasionally see fan-run replacements but I'm curious about singleplayer and offline mods. It seems like more of a niche as not all multiplayer games would work as well without other players.
I tried SPT a few months ago (with additional mods) and it was enjoyable especially if you like tarkov. There's a mod which makes the maps connected, where different extractions lead to other maps. There's lots of things you can do to make the singleplayer experience better.
Although I've ended up playing the GAMMA modpack for STALKER Anomaly instead. It has what I look for in tarkov, and then some.