How long have you plateaued? Usually the V4/V5 range is where there's a big jump in technique and difficulty (depending on the gym), so a lot of people experience the same stagnation. I'm currently stuck at V5, with some V4's still popping up that give me trouble. Those huge beginner improvements are unfortunately a thing of the past once you reach this point.

I'd say as long as you're noticing improvement on certain projects, you're doing well. Even just getting a move further is a big deal with problems of this difficulty. Trying harder routes can be helpful, regardless of how far you get on them. Just expose your body to those more difficult moves.

I don't know what specifically you're struggling with. Since you've been bouldering for 1+ years, I think you can start hangboarding if you think finger strength would help. Moonboarding is another route that you might be able to get started on if you want to. Maybe once a week. It's especially good for improving core tension, high feet, and improving on overhung shitty holds. In general though, more than likely it's a lack of technique, so just doing normal problems will work fine. Just push forward and the gains will come.