Build an MVP that can acquire actual paying customers, and the growth rate/ROI will end up making the decision for you.
If you still depend on a job for paying the bills (which means that you don't currently have access to lots of capital), then selling your product BEFORE you have built it is the only option I would personally consider going forward. And if those prospects are just entries on a waiting list. Our product was proven in a cheap 250€ ad test and requested by agencies before we all quit our jobs.
Missing product market fit is the top 1,2 and 3 reason for failing to build a startup.
They used to let us file requests to confirm whether or not it does compete. They stopped answering and now just say "use your best judgement", which is a nice way of saying "We'll sue you if we need to".
If you have a problem which is itching you to solve by building something, Then it's probably the best way to get started without quitting your day job. If others are willing to pay for your solution for solving their problem, Then you might have a startup idea(which actually can work out).
P.S. In case you want to take a look at some problems which needs solution (or) to check if there are others who want your problem to be solved then I run a platform for that - https://needgap.com.
I am beginning to see some real traction with https://www.budgetsheet.net and it's currently at ~$500-800/mo, but I can see a clear future where I will likely have to make a choice to leave my day job before it is making anywhere near my FT salary. It's a bit difficult to know where the line is.