There's a niche service nowadays called "SaaS-as-a-Service" where an entire SaaS ready to use and customize is priced to you. I didn't use one myself but [1] is a good example.
This is the correct answer. If you're trying to validate a business idea and this kind of thing is not your core competency, just get an entry-level plan with a tool like this. I'd also highly recommend you sign up for a CRM tool like HubSpot to capture leads, send marketing emails and do landing pages. It does all that kind of stuff adequately well for a small business. WorkOS is another option although I've never used it and can't endorse.
Save 4 hours by using their payments system instead of building one? Who's building payment systems in 4 hours??
It won’t use the newest framework of the month JS library in a few months, but that’s okay. Validating an idea quickly without much fuss by having a boiler plate done for you is key. Just move sooner rather than later to something you understand once you get to a stable point.
This requires I assume a bunch of paid subscriptions to third party APIs?