Tips for freeing throttle bodies stuck to head due to old rubber boots

I am trying to perform a service on my motorcycle. To do this I need to free the throttle bodies so other engine components can move.

The motorcycle is 15 years old. I had expected some difficulty wrenching the throttle bodies off but I did not expect it to be this difficult. I have followed the manufacturer service manual to the exact step and all parts needing to be loosened have been loosened including the lower clamps on the head that hold the boot in place. The boots are supposed to come off with the throttle bodies. There are additional upper clamps that I cannot reach that clamp the throttle body itself to the rubber boot. They are faced in a way that suggests to me (paired with the manual not saying anything about them) that they are not to be messed with.

What I have tried:

1. Silicone lubricant spray in the crevasses where the boot meets the throttle body. This didn’t seem to clear out any gunk and didn’t really improve the situation. WD40 would’ve been ideal but these are rubber and WD40 and rubber don’t mix.
2. Heat gun on high setting. I applied the heat gun for ~15 seconds to each boot and this did free them slightly. However, it was not enough to get them over the lip that is holding them in place.
3. Messing around with a thick flathead screwdriver to pry them from the bottom. I marred up the outer corner of one of the boots and decided to not proceed lest I drive the screw driver directly into the head by accident or damage a boot beyond repair.

In each of these cases I stood over the bike, center to the throttle bodies, and pulled, pushed, and twisted them any way I could. I am actually very tired from the process.

I am posting hoping that someone with more experience than me could give me some advice. I’ve thought about ways to gain mechanical leverage over them. For example, I could rig up something to screw into the throttle bodies where they mate with the airbox and pull them that way. I am afraid, however, of damaging the brittle aluminum threads with a stronger metal/too much force. I’ve also thought about using tiedown straps hooked to ladder for more leverage but the fuel rail is very frail and I dont want to wrap anything near it.

Are there any good tips for getting these things off?