No idea about payments, as that can heavily depend on locality (Stripe might be an option as they expand to more countries, though). For Authentication, I'd sa

No idea about payments, as that can heavily depend on locality (Stripe might be an option as they expand to more countries, though). For Authentication, I'd say go with Keycloak. Yes, it's a reasonably big java program. But it also covers a lot of authentication stuff transparently for you - your application can just use whatever OIDC client library you have, and you can centralize everything in keycloak server, including integrations with external authentication providers (Google, FB, random OIDC, client's internal systems for B2B SaaS, etc.) and you have low chances of waking up and finding out you're going to rebuild your authentication stack because you coded yourself into a corner. Authorization is a trickier case, but so long as you make it so that you can get "role" or similar attribute out of keycloak into your applications, you can start with hardcoded role names (possibly with extra attribute describing organization etc.) all the way to something complex like Open Policy Agent or even Zanzibar. I'd probably stick to reasonable written docker containers and vendor-provided SQL database as backing store + graphql interface (I do not use GraphQL myself, but I recall that at least for postgres there appear to be reasonable options?). Use what you know to reduce rewrites. With docker container, you can go from crappy VPS with docker compose (not recommended, but it does work, it's just crap - been there done that), through ECS (personally I dislike it, but you're already working with it, maybe AWS tax won't hurt you too much?) to kubernetes deployment (most complex to begin with, but scales developer resources well thanks to good abstractions, might be of interest if you find a need for deployment flexibility or find yourself in need of extreme cost efficiencies in hosting). If your code runs well with such setup you can probably also just run on Heroku if the economics fit. Standardize on observability stack, whether paying for hosted one like honeycomb, to setting up prometheus+loki+grafana. You're already reasonably successful under my metrics ;) but remember that while setting up some of it might take more time than just hacking it up, handling it well has compounding interest result on later development. Check carefully what will be your budgetary effect for "buy vs. build", both in terms of coding from scratch vs. deploying existing OSS product, and in terms of hosted vs. self-deployed. I have no idea about your budgets, but I do not joke about AWS tax lightly :)