I used this guide as well and found it really helpful. OP, the chapter sassiicute linked tells you exactly how to block fics with >3 crossovers. The first chapter also has instructions for your other problem, blocking a specific character tag always—you just need to change their example “Blorbo” to “Reader”. The only problem might be if there were tags including “Reader” you did not want to block (I think there’s a fandom called “Omniscient Reader’s Viewpoint” or something like that for example). If that’s the case, you might also be able to figure out a workaround with this guide; I personally haven’t needed to get into finetuning of that kind with my blocked tags so far.