The word "no" can be very effective. Remember that you control the type of work that you take on.
I have a small side gig building "controllers." By controllers I mean devices that are typically arduino controlled and use peripherals in the arduino ecosystem. They span a very wide range, but are typically very feature-limited. e.g., I have a client who is converting massage chairs to be pay-per-use.
As you noted, it's not easy to keep a service-based business from growing to take over all your time. I manage it by keeping the feature set clearly specified and working on fixed price.
Want to add a feature we didn't discuss? That's another charge. My niche is taking on very small projects that are too small to move the needle for a full-blown engineering services company (I've worked for two) and I always work fixed-price, so I need to be very aggressive about scope creep.
Project scope keeps growing? Either tell the client that it will be a while until I have time to complete it, or, more frequently, that they will need to find someone else. This is pretty easy to say because as mentioned above I'm clear about only taking on small projects.
I've had people who basically want me to be their engineering department. That's a hard "no:" I simply don't have the time.