Strength / Weight Training

Could also try getting a band to help you to do more reps
Practice hanging for grip strength

18 is amazing. never been at that level, but when i was a bit... ahem... lighter, and did a lot of pull ups once I could do 8 I did a mix of higher rep bodyweight sets and lower rep weighted sets.

There's also the russian fighter pull up program - https://www.strongfirst.com/the-fighter-pullup-program-revisited/ - you could do the weighted variation.

But yeah, add weight at this point I reckon. if you don't fancy the program above you can find your 2rep max (2 rep max cause easy to accidentally cheat a bit on rep 1 of a chin/pull up) and try to do 3-5 sets of 5 at 85-95% of that weight (where 100% is your bodyweight plus whatever you hang off you) you'll be in a good spot.

First of all - be careful to not wreck your elbows.

That said, there are a handful of things to try: add weight - fairly straightforward.

Train different grip positions/width etc. Palms facing/facing away, narrow grip and wide grip.

Do offsets (one hand higher than the other). One of the easiest ways to do it is by hanging a towel over the bar and using that as the lower grip. Good work on your grip too. You can also swap the towel for a resistance band, but it’s a lot harder than with a towel as it’s basically a one arm chin-up minus whatever assistance the band can provide (which often isn’t much).

Do negatives - jumping to the top position and doing a controlled lowering, do these when you can no longer complete a rep normally. You can also do them with 1 arm for a particularly savage workout, NB doing one arm negatives is fairly traumatic so do it as the main part of the workout rather than at the end.

Superset bicep curls before doing chins so that your back is having to do more of the work.