Nowadays, I seldomhardly play video games, but I sometimes do PUBG mobile, which is a battle royale game with 100 players. Nowadays, I hardly play video games, but I sometimes do PUBG mobile, which is a battle royale game with 100 players.

"Seldom" is actually the perfect word to use here, but it isn't used very frequently in spoken English. It is fine when used in written communication :)

This game has a variety of modes, but I mainly play in quad mode. This game has a variety of modes, but I mainly play in quad mode.

You would normally say "I play IN quad mode" or "I play IN solo mode", because you are playing the game within the environment of that particular mode.

It needs four players, so I play this with three of my friends. It needs four players, so I play this with three of my friends.

It is my stress reliever. It is my stress reliever.

This comment makes sense, but we would usually say something like "It's a brilliant way to relieve stress" or "It's my stress relief [for the day/week]".