I'm unfamiliar with the Forgotten Realms setting. TBH, this is my first experience with it. I don't think it's connection to DnD is shallow at all. There are some key differences that come from it being a single player video game, like having full control over the entire party, and there are home brew rules, but what DnD group doesn't have home brew rules? My group plays with lingering injuries. Larian has extra bonus actions, environmental effects, super jumps, etc. At the core of it all, it still feels like DnD to me.
Sure there’s homebrew. But if I had a DM who homebrewed as much as Larian did for 5e at the table, I’d leave. This isn’t core 5e with homebrew. It’s core DOS with 5e mods.
I didn't play DOS either, so I can't speak to that. I have to assume that core DOS is a lot like 5e, because BG3's gooey 5e center seems pretty evident to me. D20s affect the same stuff, add your proficiency and ability modifiers. The most radical change I've seen was done to the rogues. No expertise, which is something worth complaining about, but it doesn't ruin the game for me. Thieves gain an additional bonus action, as opposed to the stuff it gets in 5e. No sleight of hand bonus action. Again, worth speaking up about if it bothers you, but for me it's not a deal breaker. What am I missing?