Here's why I'd pick it:
1. One language for the whole stack. It's an MVP so the front and back are both changing fast, with changes made by the same person, and you don't want to context switch all the time.
1a. Javascript as that language because it's the most flexible. And with TypeScript you can gradually opt into type constraints where they help you.
2. An opinionated full stack like Ruby on Rails that integrates all the pieces I need to ship. When you assemble yourself, even small version bumps or outdated tutorials can send you down a rabbit hole, stealing time from testing PMF. You want someone to validate a combination in which all the pieces work together smoothly.
2a. Redwood.js closest to be being the Ruby on Rails of JavaScript. I've toyed with it before and got up and running quickly. Its opinions took a lot of unimportant decisions out of my hands and let me focus on what I was trying to produce. I didn't get so far as to confirm this but it also seems to make it easy to gradually drop the scaffolding if the opinions and simplifications ever get in your way.