What is the general 'good practise' or consensus (if any) around environments and environments promotion? Do you use dotenv (or similar) or use the stack as an environment? I am tempted to go for the second option and name everything after the stack.

That's what I do. It doesn't work if you want to promote an actual deployed environment (e.g. the current "dev" resources are accepted by testers and become the "test" environment, but that's not a common practice. If you want to promoted a branch or tag to a new environment, then stacks work perfectly.

yes in theory we can even do branch deploys, that's a great idea, thanks