For the database, anything that's managed - I'm partial to SQL server just based on my work history, but Postgres is nice too. Even the expensive ones are like $100/mo for the lowest production tiers, if you're bootstrapping with a W2 job that's reasonable for 6 months while you try to find some level of traction.

For the backend, probably default to JS or maybe TS lambdas/function apps. Quick to write, quick to deploy, slap an API gateway over the top and you're ready to go.

For the front end, for a true POC I have a lifetime/full Creative Tim membership. So I'd just grab a dashboard and landing page template and start plugging in calls to the API gateway.