I also like the Follow approach best.
Follow the HolosIndex bot gives explicit consent "please index my public posts". HashPub by @evan follows a similar method, and also https://anagora.org by @flancian works on that basis. Give a nice acknowledgement after the follow request, and perhaps an unfollow removes you from the index again.
I find this a better mechanism than the magic hashtags in profile approach, where there's already tight charlimit in many apps for the intro. You also have (or had) this with https://fediverse.info when adding #fedi22 to ones profile.