The default tooling isn't great. Tools like CrowdAnki [0] and Anki Deck Manager [1] make it possible to manage decks in plain text. It's also helpful for collaborative deck development. Unfortunately I haven't been able to find many examples of decks managed this way online, but there is a relatively high quality geography deck worth looking at [2].

[0] https://github.com/Stvad/CrowdAnki

[1] https://github.com/OnkelTem/anki-dm

[2] https://github.com/axelboc/anki-ultimate-geography

I think that's what "Tools > Create filtered deck" is for. Assuming the cards are appropriately tagged, "tag:hsk5" should be enough.

For more complex operations Anki doesn't support directly, I usually just export as text instead of dealing with the database directly.

> I was trying to edit the full hsk1-6 flashcard set to just show hsk5.

> But I didn't manage to find the right tools.

This is by far the worst part of those decks (which I am also using). You are literally encouraged to just browse the cards and delete the levels that you're not using. It's really not user friendly.