How can I add breakers for a new HVAC system to a full panel?
Option 2 is probably not an option at all - that setup is almost always for a special-rate storage electric water heating tariff and is only on for a few hours when demand is low overnight (which is why it's a special lower rate) or else it's switched off whenever loads are high (a different scheme, similar concept.)
For my utility, you can keep them if you have them from decades ago (when they had a nuke plant idling away all night), but you can't get a new one done this way. You certainly cannot add to it, (for my utility) you don't own the water-heater associated with it - it's part of the overall deal, they own it.