Which mod (s) would you recommend to me (Minecraft Forge 1.12.2)?

The mod (s) should contain: something where you have to research things or

Dungeons (not just vanilla stuff) or

really cool loot and bosses.

I would recommend a modpack here.

Thank you. Which one?

No idea, go search in the Twitch Launcher, you can narrow it down by category, e.g. Adventure, which is probably the genre you are looking for

OK thanks