The app will manage hardware design documents for 50 customers.
fastapi/Python
Postgres
Celery for tasks
Redis for caching
React for frontend (Tailwind)
Hosting: DO app platform, heroku, k8s on AWS
Sentry for error tracking
Stripe for payments
EmailOctopus for newsletter
SendGrid for transactional emails
NameCheap for the domain
auth: Auth0 or Amplify from AWS
AWS S3 for storage
Github for code
Trello, Slack, Google docs
If/when you need to start "real company stuff": AngelList Mercury Ramp Rippling HubSpot
I've been pretty impressed with all of those.