We'd love to help with your user management at https://clerk.dev - stack-agnostic Sign Up, Sign In, and User Profile Management - oauth and 2fa included out of the box
Others:
- stripe.com Atlas for incorporation
- vercel.com for easy frontend deploys
- render.com for easy backend deploys
- pilot.com for taxes
- stripe.com for subscriptions
- sendgrid.com for email
- orbit.love is good for tracking community growth
- discord for actual community conversation
- best marketing for us has been word-of-mouth, thankfully driven just by doubling down on product and support