The groovy platform is definitely going away, there have been official announcement to that extent, we just don’t know when and it could be quite a while yet.
You didn’t mention what technical background, if any, you already have. Do you already do any coding in any language? With the new platform, you can code pretty much in any language you want because you will host your own smartapps and communicate with smartthings via their new API.
My personal suggestion for now, though, is that you just start with webcore. It’s essentially a scripting language for smartthings. Very powerful, and there are lots of people who will be able to help you with it. It has its own forum.
As far as writing automations which include multiple platforms, it all depends on the integration possibilities for each platform. Xaomi does not offer very many, they basically assume that people will be using their devices with their gateway. I haven’t heard of anyone trying to combine a Xaomi gateway based system with a smartthings hub-based system, but hopefully if someone has they will chime in.
There are other hubs/gateways that do have open APIs or have Ifttt channels, and then there are things that you can do, just as you can work with a smartthings hub and a Phillips hue bridge. But it comes back to the first rule of home automation: the model number matters. It will be possible with some platforms and not with others.
As far as getting started with a new platform, see the following link: