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.
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 think we will start seeing some of the first of these released in the coming week or two.