All SaaS that I have seen have some sort of user tracking (mixpanel, amplitude, posthog, ...) which tracks pretty much everything that users do. This can be done completely server-side since you can track all API calls and attach some user metadata to them.
With client side tracking you can do even more, especially when it comes down to UI optimization. Not too big of a fan of that to be honest.
Besides that, user interviews are always great and as a CEO or product manager I would also make an effort to talk to users and take time to gather feedback and just listen to their problems. Staying close to your users is important. Who doesn't like to be heard? However, it's one of the things a lot of startups struggle with or don't see the value in.
Last but not least, if you have only a few users talking to them is the most important thing. Analytics only really work when you user base is large enough. The way to go is to define a set of funnels (what do you expect users to do in what order) and see if these funnels work. Besides that, there needs to be some kind of funnel discovery (what are your users actually doing in what order). The second one if important because it tells you a lot about intentions and gives you a great idea of who to talk to.
For other data collections, I use form within the analytics platform, something like [0], where I store metadata attached to a user session.
[0]: https://docs.uxwizz.com/guides/useful-examples/feedback-form...