Professionally I work across multiple OS’s and architectures so all .NET languages are kinda no-go’s

.NET has had support for Linux, Mac and ARM for a while now.

If you are using OctoPrint there is also a Home Assistant integration for that.

Yes it can, though it is easier to set some things up with the built-in addons. Most addons can be set up independently as docker containers (like z2mqtt or node-red) but may require additional configuration.