Our cat loves us too much (advice needed)

I'm going to start out by saying - we love our cat. We adopted him as an adult a little over two years ago and he is the sweetest, most affectionate young guy you can imagine. He is also super energetic, which is great when we want to play with him.

All he wants is to eat and be loved all the time, in that order.

We also have a pretty small apartment and both work long hours, so he is alone almost all the time, couped up with nothing to do.

This is my first cat, and my SO's first cat who is an indoor only cat - so we're not quite sure what to do.

When we get home we each try to play with him for a while to give him attention / exercise / burn off some of his energy, but it is boundless. We feel bad for him, but lately he has also been making our lives hell.

We live in a studio loft, and our bedroom is in the loft. Since there is no door we got a gate, which works well to keep him out, but every night he just stands on the top step and rattles the gate trying to get our attention. It is super loud and keeps us from getting restful sleep.

He has also gotten into the habit of doing drive-by attacks on my SO (he doesn't do it to me) from time-to-time when we haven't given him attention yet. On some occasions these have left long, nasty scratches that (I assume, based on appearance and testimony) hurt quite a bit.

We love this cat but want him to stop turning into a devil kitty. We have tried taking him for walks but as soon as he hits the ground with the harnass on he tries to run away (with a near disaster one time when he slipped it - my (not actually a) dad reflexes saved the day on that one.

We've thrown around the idea of getting him a companion, but are concerned that we would just be stuck with another cat and it wouldn't actually help solve the issue.

Would love any advice on the best way to fix this.

Sincerely, At Wits End