First, I’d like to know if it’s necessary for me to report posts if they’ve already been reported on Blahaj. Like, is it just a federation delay that causes the bad posts to still be visible for a while, or is my reporting necessary to get them removed?

It's complex. If an admin does an instance ban and removes comments, then the removals don't federate. If a mod removes the post, or if an admin manually removes a post, post by post, the removal federates. We have a bot that works around that problem though, by manually removing posts one by one, so the most helpful thing you can do is make a post saying "Troll account, please remove all posts" or something like that, rather than reporting every post.

Also, I wonder if I made a mistake by boosting a few threads recently. Like did I accidentally get the attention of these transphobes? Should I refrain from boosting?

Nope, it was nothing to do with you. This is just some kid that got banned from a conservative community on one instance, so he created his own on another instance. That community got banned because he was transphobic, and so, he's having a tantrum, and doing more transphobia.

Again, I hope it’s okay for me to post this here.

More than ok. And I apologise, I sent you a DM about reporting posts, without realising you'd made this post here!

And, notably, we have now defederated from lemmy.today.