I have separate automations for turning on the HVAC and turning off the HVAC. When the “Turn HVAC On” automation gets triggered, one of the actions is to enable the “Turn HVAC Off” automation. One of the actions on “Turn HVAC Off” is to disable itself.
Does that get you what you want?
I have all of my automations for HVAC in a package (https://www.home-assistant.io/docs/configuration/packages/). Maybe it’ll seem more tidy/less cluttered if you do it that way?