OP asked for games that too few people have played, not for hidden gems. That is not the same. For example, compared to the 500.000 reviews Baldurs Gate 3 has on steam, Divinity: Original Sin II, despite also being one of the best rpgs, has only 150.000.
FTL, which is easily in the top 5 of the best roguelites ever made has only 50.000 reviews, which is very little compered to Hades for example, which has 200.000.
Into the breach was never really popular, it has only 15.000 reviews on steam.
If you ask most gamers about those they usually have never heard about them or at least have not played those themselves. I would not call them particularly hidden but I think they deserve to be played by more people and that is what OP asked about.
You will find one but it´s all manageable. I recommend focusing on one faction and one or two unit types at the beginning and learn just those tech trees first, to avoid getting overwhelmed by the abundance of units and go from there. Vehicles and/or Bots are good unit types to start with. Just let your team know what unit types you will go for at the start of a match, choose a fitting starting position and it should work fine.