I find that Piped has a fairly decent “algorithm” for suggesting videos if you are already on a video. For example, “trending” doesn’t do anything for you, it’s effectively the same as not being logged in. Feed, once you have your subscriptions, is the same but for your channels.
But once you’re on a video of one of your channels, the suggested content is pretty good. If you go to SEA or PBS Space-time you get other space videos. If you go to electroBOOM or Practical Engineering you get other electrical/device type videos, game channel and videos and so forth. And it’s pretty spot on, whether your watching let’s plays or reviews or philosophical discussion around it, it usually keeps it fairly in line with whatever that channel/video topic is on.
It’s not perfect, like if you want the full broad spectrum algorithm. However, it’s good enough for me as I tend to find that if I’m in a specific rabbit hole anyway I’m not really trying to find the diverse content, but continuing down what I was looking for.