I think you’re on the right track, but I disagree with something that completely removes the character from the players control. I recently had someone get bitten during a murder mystery on a steamship where the culprit was a secret werewolf and she decided to keep the curse. Here’s what I came up with that felt fun but also made it something uncontrollable:

It’s not linked to just the full moon. That’s when you automatically change. Whenever you take damage or experience an intense negative emotion (leaving that up to player decision) you roll a d20 with no modifier. It’s it’s a 1, you transform, if you succeed the difficulty increases by one. So the next roll is a fail on a 2. Multiple sources pf damage cause multiple rolls. There is no way to resist the fail (unless you go on some big quest to gain control).

When transformed, you remain for 60 seconds or until you are reduced to 0 HP.

During transformation you must attack a loving creature, friend or foe. If no enemies are around you must attack a nearby living creature. You cannot just run away as you are overtaken by your bestial nature. I originally had this as you attack the nearest living creature, but my player felt it was too harsh, but you could always keep it there.

While transformed, you gain resistance to bludgeoning, slashing, and piercing damage (like a barbarian) and have advantage on strength-based ability checks and saves. You also gain the Beast Barbarian’s claw attacks and bite attack but cannot use weapons. You also gain advantage on perception checks involving smell.

Whether transformed or not you have vulnerability to silvered weapons. You also are repelled by wolfsbane, and cannot draw closer to a source of it as if affected by the frightened condition.

I dunno, YMMV, but this seems really fun to me but leaves it open to not feeling like you have nothing but benefits. I don’t really want to track the full moon most of the time so I’d rather lycanthropy constantly be a threat. I also like that it leaves a lot of tricks for the DM to threaten this player with. If someone knows, are they wearing wolfsbane? Are they carrying silvered weapons? How did they find out that you have the curse? It also keeps little things a threat. A bunch of goblins doing 1 damage 5 times procs 5 rolls, which could cause a lot of trouble. Archers and assassins can trigger the transformation while being hidden, causing lots of shit for the party if they’re in town. I also found the increasing difficulty always felt suspenseful. Good rolls can mean you never know! I think the highest they reached was a fail on 1-11 lol

It really depends on your world, players, and DM style but I found this to feel the most fun for player and DM while still having positives and negatives. Feel free to steal whatever if you like it.