Hmmm, for buying automatically, you might be looking into Bandcamp Clubs. You pick one of such "clubs" and you get one album into your library each month that you can keep forever.

For downloading, there's Batchcamp.

I personally don't mind visiting the website whenever I have some spare time each month to get something. I also have a task that reminds me of Bandcamp Friday (spoiler: it's almost the 1st Friday of each month). In the months when there's no Bandcamp Friday, I just mark the task as complete and move on.