"Does everyone else do all this back end user/payments stuff from scratch? Or am I missing something..?"
Yes, you're missing something :-)
It's smarter to leverage existing Rails Boilerplate Code to bootstrap your product work.
"Boilerplate Code is any repetitive code that shows up again and again in order to achieve the same results.”
Why reinvent the wheel?
- Sign up and login
- Stripe/Braintree integration
- Account and profile management
- Create and manage subscription plans
- Website management i.e. Title , page creation etc…
Combining all the above boilerplate code is a fantastic starting point that includes the pieces, out of the box, to hugely reduce cost and speed up go to market time.
"I'm mainly interested in 1. ease of learning (because I'm self-taught), and 2. speed of development (for building MVPs)”
Here's a short list guaranteed to be easy to learn and speeds up development.
RailsKit is SaaS product that focuses on Accounts and Payments. It supports a wide array of gateways like Authorize.net CIM, Braintree, Payment Express, Stripe, and TrustCommerce, all via ActiveMerchant. It has fast User Account management and it's easy to customize.
2. Bullet Train
Bullet Train is a SaaS service that bills itself as Saas-in-a-box. Bullet Train saves you weeks of development by starting you off with all the features that are the same in every SaaS, so you can focus on what makes your app unique. Very full-featured and worth checking out.
Suspenders is a gem created by Thoughtbot, a leader in the Rails community. Suspenders is the base Rails application for everything new at Thoughtbot, and it's already integrated with CircleCI and Heroku for quick deployments. Not specifically focused on SaaS, but easy to configure it to be.
4. Rails App Composer
Rails Apps Composer is a Gem that helps you create a started app from the command line as well as allow you to create your own "recipe” for a boilerplate application template. There is no SaaS recipe out of the box, but it's super simple to create one and reuse it over and over again.
5. Rails Application Templates
Last but not least, Rails Application Templates is an in-built Rails mechanism for setting up your own re-usable boilerplate templates. The primary focus is on Gems and Initializers, not SaaS specifically, but it's very powerful and well worth investigating.
Using anyone of the above will save you weeks of work and high development costs by simply starting off with all of the basic features every SaaS application needs.
Now you can focus on providing real business value to your users, and immediately start charging for it.