Reverse Engineering Web Application/Service

Hi Guys!

Is there a known harness/workflow that makes it easier to Reverse Enginerring a Web Application/Service thats behind a payed subscription?

My current problem is that I use a service that costs me quite a lot of money but still does not have all the features or tweaking options I need.

Now I'm asking myself, if it would be best to describe every feature myself or if there is a way to let a LLM "explore" the Web Application/Service by itself and write it's own notes what it needs to code.

Did somebody here do something familiar and has some tips?

Thanks in advance!