So making users look into the promoted tab instead of hot or trending would already go into the direction you proposed, I guess?

Or, sliding promoted content through the promoted feature into trending for a price would work well as well. If promoted runs a bidding system (which I think it kind of does). Bid your way into the top spot and appear in trending in the first 5 posts. 2nd spot in the top 10 etc. (arbitrary numbers). It could have a feature that for lower amounts you could buy into a tag trending position.

This would take away the need for bidbots to get into trending and burn the SBD in @null

It wouldn't get rid of the bidbots but there would then be a clear separation of the motivations of why to promote the posts. Bidbot users would only have the claim of profit. I have no problem with people making profit but, be honest about it and don't hide behind the various masks claiming for engagement.