So my friends and i started a group from scratch, and i ended up as the dm. We've been playing for a while now, from lvl 1 to 4 on a hombrew campaign. Thing is I really struggle to balance boss encounters, either my party steamrolls through them and i gotta improvise something else or i have to nerf the enemy so i dont have a TPK. Like how do i know how many minions are too many or what CR should a boss be?

Is there a rule of thumb, like "CR 2 levels higher than the party and two minions" or am i just lacking the experience to have that 'feel' for what is going to happen?

For a little context, we're playing 5e, the party is level 4 rn (4 or 5 players per session), with a couple of uncommon magic items, and my campaign revolves around cunning mind flayers with an Elder Brain as leader trying to conquer a kingdom from the shadows. My players are heading to an underground ruined forge, and i have already planned encounters with cave creatures and some constructs, but cant figure out what are they going to fight at the end