Not Foss but does the job and well https://www.macrodroid.com/

No

Just request the ones you want here and it will start syncing them. Make sure you subscribe after the communities are made (takes a few minutes) or the dev will disable the bot on those communities. The more people who subscribe, the more often it checks for updates. https://lemmit.online/c/requests

Check out lemmit.online instance. Let’s you request subreddits and it automatically copies them to a lemmy community