For people in a similar situation, I'd strongly suggest giving Phoenix (on Elixir) a look. It's a "boring" Rails style server side rendered web framework, but with a few extra powers:
* Elixir is a great functional programming language
* When your backend needs to do more complex or longer running work it's just more Elixir rather than complicated architecture involving task queues etc. (because you have the concurrency of the BEAM to manage it)
* You have options like LiveView for your frontend. You don't have to use it, but it's there.
I agree with what you said about testability for Lambda style architectures. How do you spin up an offline version of some or all of that to see if it works? The BEAM world is nice for me because you can run a similar architecture of separate things (albeit all in one language), but have a much better testing setup.
* Elixir is a great functional programming language
* When your backend needs to do more complex or longer running work it's just more Elixir rather than complicated architecture involving task queues etc. (because you have the concurrency of the BEAM to manage it)
* You have options like LiveView for your frontend. You don't have to use it, but it's there.
I agree with what you said about testability for Lambda style architectures. How do you spin up an offline version of some or all of that to see if it works? The BEAM world is nice for me because you can run a similar architecture of separate things (albeit all in one language), but have a much better testing setup.