If you're looking for something that fits the motivation (including "give me the developer experience of the 90s"), but uses a language you probably know already (JavaScript), check out https://www.convex.dev/.
Now you hear consultants telling you Kubernetes on top of whatever cloud service they are shilling is the one true way, or some other convoluted mess built on top of a Jenga tower of Cloud Formation and Terraform configuration files. JSON has replaced XML but otherwise, same old.
I found only frontend related code https://github.com/get-convex , is backend somewhere there?
Meteor has all of frontend and backend Open Source.