Here is my product: https://matchkraft.com/. It is a marketing toolkit. I think you need to ask for feedback, asking why users don't want to pay for a premium subscription. So far, you are getting new users that is a good sign you are going to the right direction. Maybe, who is your ideal customer or maybe your product is ok but need more visibility (maybe paying ads). Try to get active users. I really want to... Source: 12 months ago

Calling a 3rd party API: There is a complete ecosystem providing "google-sheets-as-DB". I personally tested and recommend https://sheetson.com/ but there are a lot more with free tiers https://sheetsu.com/ https://sheety.co/. Source: about 2 years ago

Some professional software such as TrueCoach or Trainerize can help to make your clients experience much smoother and make you appear more professional. Some kind of scheduling system such as Calendly can also be very helpful. You should understand how to program effectively for your clients. Shadowing other trainers can be very enlightening too. Source: almost 2 years ago

I've used Google Sheets in the past, but I currently utilize TrueCoach thanks to it's more professional user interface and various other quality of life features. Source: about 2 years ago