Tools add value but don't be too obsessed with them including KPIs. You need to ensure that as a business, you are getting the type of clients you want. Then, you need to ensure that those clients are happy with your product/service. Listen to them and enhance your product/service accordingly.

Also learn from your mistakes. If you had that one really bad client, look at why there were bad to you. Avoid those mistakes next time. Document it, tell your team whatever. If releasing a critical feature broke things in production, again learn from it. Why did that happen and what can you do to minimize the risk of that happening again ?

If a client leaves, ask them why they left and what could have been done to keep them with you ? Sometimes, the answer is just "I don't need this anymore" but a lot of times you will hear things like "You guys don't have this thing that I really need" or "I need better support", "too expensive for me" etc. Anecdote: I have been successful in retaining a few clients by offering reduced pricing but with reduced features as well because they didn't really need all that fluff.

At 100 users, I'm most concerned with how quickly/easily I can get them on the phone, then how much they talk when they're on the phone.

At that stage, I'm still proving out an idea so having focused users who not only like the app enough to talk to me but who have ideas on how the make the app better is a sign I'm on the right path.

If getting in touch with my first 100 wasn't an option, I'd look for signs they were actively using the app. I would want to find out if the app was indispensable for my users. Sadly, I can't think of a better way to collect that than by picking up the phone.

PS - If you find talking to users tough, reach out and I'd be glad to help you write a script. It sounds phoney, but a good script will help you get through your first three calls. You'll be rolling after that!!

It totally depends on your business model. Is your expected ARPU 10, 100 or maybe 1M?

I'd personally focus on a KPI that tells you something about how much your customers like your product, and can't live without it. Aim for turning even a few of them into evangelists. Retention is the main force driving that.

But once you get that deep, it's just as easy to implement a full AARRR funnel that will give you much more insight.

It really depends on your app. I would say the most important metric is the number of customers willing to pay for your product happily.