A model I've uses is one of risk. Whatever your idea is there are going to be a number of potential reasons it might not work out.

For some ideas the biggest risk is the market, does anyone want this? For others it might be the technology, is this even possible?

In a lot of cases you can do things to reduce those risks and give yourself more confidence to jump ship and go all in. That might be talking to people, it might be reading academic papers. In some cases, but honestly pretty rarely, it will be building something. Most of the time that can come later.

Not dismissing any of the great advice in here already around non-compete, company IP etc. most of this kind of work (research, conversations etc) isn't something your employer can "lock down" as concretely as an MVP. Plus if it is tangential to your current job and your idea doesn't work it, you may still end up better off from having learned something new.