People normally have the reverse complaint, that the algorithm makes them miss content.
Huh? Mastodon certainly has hashtags. Likewise, there are posting groups. More implicitly, different social groups tend to cluster together and re-boost interesting things.
Like, to be clear, fedi isn't going to be the right solution for everybody (or necessarily even for most people), and it's totally valid to dislike it or prefer something else. But it certainly does have organizational tools; they are just differently shaped.
I don't know whether it helps your use case, but Mastodon has a "Lists" feature, which allows you to group the accounts you follow into separate realtime threads. For instance, one list could be for small accounts from which you don't want to miss any post, while another list might be for high-volume daily news accounts (and IIRC, there's even an option to "hide accounts on this list from your home timeline", so you could use lists for the noisy accounts and have everything else in the home timeline).
HackerNews posters are not the typical Twitter users.
(Just to be clear I personally hate the algorithmic feed but there are lots of use cases for it that just don't align with my Twitter usage)
I don't personally notice a lack of engagement. Maybe not everyone migrated, but I feel like the most important people have.