I'd suggest Mastodon to anyone looking for a new social media platform to join, but I can't suggest Lemmy/KBIN as they don't have legitimate blocking in place, as theirs is simply the same bullshit Reddit like "masking" where you can no longer see who you blocked, but they can still see, and reply to whatever it is you post, which of course is rife from abuse from the people you want to distance yourself the most from. Until there's legitimate, you no longer exist, blocking on Lemmy/KBIN I would suggest anyone else use them. (to say nothing of all the errors in KBIN from notifications, to blocking, to posting, to seeing comments, the list stretches endlessly)
My problem with mastodon is the same one I have with Twitter. It’s very easy to get up and running. You can get a feed filled with nonsense to read but if you’re actually interested in any topic, it’s really hard to get a feed that matches your interest. 
That's why I could never get into Twitter and now Mastodon. I don't give a shit about who posts, I only give a shit about the topic. I don't want to follow people. I want to follow topics. And hashtags just don't cut it.
Fully agree.
I'm one of those people who migrated to tumblr after the Reddit exodus (tbh I did have tumblr beforehand, but my usage increased a lot when the API bullshit happened). After using it for around 10 months I've come to the conclusion while yes, it is good for fandom related content, memes and some other things it's simply no replacement for reddit (it's actually more of a replacement for my fandom instagram to me). Yeah you can follow hashtags or topics, but tumblr is too niche of a platform and a lot of the topics I care about the average tumblr user doesn't even think of, or at least not to the extent I do.
If you want a reddit replacement Lemmy is simply your best option. If you want a Twitter replacement then yeah, go ahead and use Mastodon.
You can also use Friendica if you enjoy Tumblr. It has a wider degree of customization in regard to its interface, and you can also cross post or get your Tumblr feed in Friendica (provided the server you're using has the appropriate tumblr addon).
That looks really promising, I never heard of that before, thank you for telling me! Do you perhaps also know of any unofficial tumblr android app that lets me add two accounts/main blogs (ideally FOSS, but I'm willing to make compromises)?
Yeah, Mastodon is great for following people, but not wonderful for topics. I hope that Lemmy can mature to the point where it has a greater diversity of communities.