At this stage you should be starting to feel like a master of entrepreneurship experiments, things are working and you are solving problems for customers. You have all the big rocks in your business model sorted out and now it is time to make small but powerful optimisations.
https://www.youtube.com/watch?v=OtUWbsvCujM
There is a LOT to learn at this stage but a great place to start is to figure out which experiment you should be running when.
When to Use Which User-Experience Research Methods
Deep dive into which experiment you should be running and when
Rather than going through all of the potential experiments you could be running I will just leave a breadcrumb trail of useful links for you to explore when you need.