I've always found the absolute best supplement to improve strength is BHW.
BHW is Bloody Hard Work!!!!
Start by recognising that muscles do not get stronger lifting weights... or to be more accurate during each weight lifting session. They actually get stronger in the adaptation period 24 to 72 hours AFTER you've lifted weights as a direct response to the overload stimulus you've given each muscle group on each exercise from the effort you made, especially in the very last rep of each exercise, provided they have adequate nutrition (just enough protein) and rest. No overload - little or no adaptation.
I see so many gym members going through their routines of multiple sets of multiple reps and never getting to a point of overload on any exercise. If you can actually do all your sets and reps without getting to a stage that it requires real effort, then you are denying yourself optimal results.
A little-used strength training term is INROAD, which describes the adaptive stimulus created by getting to a point of momentary muscular failure - the inability to (with good form) do another rep.
There's two type of inroad: concentric (the lifting phase where the engaged muscle/s are shortening) and eccentric (the lowering phase where the engaged muscles are lengthening).
We are actually weaker on the lifting phase, so most people stop when they can no longer lift the weight (with good form).
The advanced technique that typically requires a spotter/trainer is eccentric overload, where the trainer helps you lift the weight and then take over and slowly lower the weight. With pull ups, this can be done by using a step to get to the high position and then slowly lowering yourself. But be warned that training to eccentric failure can stimulate a greater amount of delayed muscle soreness.
Good luck with your training. You only need to do it for life.