I made (with some ChatGPT help to troubleshoot) a bookmarklet which will redirect you to your own instance’s URL for a foreign Lemmy community to easily subscribe to it.
This will not correctly redirect you if you are in a topic already. It will only work if you are on the base community page. If someone wants to tweak it (or redo it entirely) to work in a bigger variety of instances please feel free. My skills with JavaScript are… minimal.
This has been the biggest hurdle so far, trying to stay on a single instance.
I know of several different groups working on this, from the main developers of Lemmy to people hacking together browser plugins to make this possible already now (i.e. interacting directly with different instances using your account from another instance). I think we will start seeing some of the first of these released in the coming week or two.