Adding insulation to sealed off attic space above master bedroom?

TLDR - older home with a Master bedroom attic space that is inaccessible. Need suggestions on how to add insulation and air seal.

Purchased a two story home built in '82 about 4 years ago near Dallas, TX. The ceiling above the first floor master is around 10 feet. The roof line slants from the peak of the second floor to the front of the house where the master sits in a large southern facing roof, which is covered in solar panels. In the summer we feel the heat radiating down and in the winter the cold air rolling down from the ceiling. We want to add insulation and air seal the space above the master, but it doesn't seem easy without some demo work as there is no access to the space. The master bedroom is always warm in the summer or cold in the winter, while the rest of the house is perfect. Windows have been upgraded.

Can anyone suggest a good method to add more insulation/air seal that space? I thought that this could be done by cutting a temporary access point and coming up through the ceiling of our master closet (8foot celling) and then up into the space above the master. Our master closet also needs needs insulation added, so this might be best approach.

I had one local HVAC team come in for a quote, and I shared with them them my thoughts. They said it would be a difficult job and recommended against it. They then quoted me 28k for removing and replacing the insulation from second floor attic w/ air seal, and installing a two zone mini-split system to supplement the 1 year old heat pump that was installed in the house. They completely ignored/dismissed the primary requirement of insulating and air sealing the space above the master and pushed for the sale, so I politely declined and ushered them out of the house.

The one professional shop recommended against it, but there has to be way to do this without sticking me with nearly 30k in work. I put a stop to my hunting due to the birth of a child and then Covid, but it is time is now time to get this stuff done. I'm not a home pro at all, but am somewhat handy. As such, I'd probably just make a big mess of things as I have no insight into issues that might arise from cutting an access hole and then patching it when done.

Thoughts or suggestions on the best approach?

Images for visual reference. I want to insulate the green painted space - across the front of the house over the master closet and bedroom. https://imgur.com/a/fbExchI