I've bootstrapped a side project before (in the B2B security space) which was later acquired. I had a _lot_ of ups and downs, but the "up" that consistently outweighed the downs was getting a new user and seeing them convert from free/trial to paid. There is something incredibly tangible and rewarding about seeing MRR increase.

Because of this, I approach side projects from the perspective of getting to revenue as quickly as possible, as opposed to, for example, hacking away at something just for fun or learning something new. Those are entirely valid reasons to start side projects, but I personally don't get the same motivation from them and have found myself quickly getting bored.

My personal preference is to focus on SMB/mid-size B2B SaaS with the goal of eventually replacing full-time job income. For me, consumer SaaS is too small - you're making $5-$20/month/user and therefore need 100s or 1000s of users to reach that goal. Enterprise SaaS is too difficult for a side project (sales cycles are too long/complex). Most mid-size companies are willing to pay a few hundred/month for a good service. As an example, if I can charge $300/month, I only need 50 customers to generate the equivalent income of a decent-paying tech job. The challenge, of course, is finding an idea that meets those requirements!