But Iran also likes the Houthis being a credible threat to Saudi Arabia and the UAE even more.

Continuing to poke international commercial shipping will obviously trigger a response. No one powerful, even China, wants non-military ships being attacked.

So they're risking substantial damage to Houthi power for... what?

Tying up naval capacity? Eh. Temporarily. And with minimal effect on Israel/Palestine.*

Boosting the price of oil? Maybe. But doesn't seem to be doing much yet.

At Russia's behest? Maybe. General chaos elsewhere and tying up ISR assets benefits Russia.

* Would become much more relevant if Hezbollah enters the war.