I’m always struggling to desolder high heat circuit boards like pc motherboards because they used high heat solder. Sometimes I can get it hot enough that I can mix in some lower temp solder but it’s always a pain. I’ve also used my rework hot air at 500* plus the soldering iron at 500* and gotten it, but that’s a pain as I only have two hands. Right now I’m replacing the Nec/Tonkin caps on a ps3. Fortunately removal is simple with a chisel, but getting the pads ready for soldering is a huge pita.
I’ve always used desoldering braid and patience. Always tinned first with 60/40 rosin core and some flux to get the heat flowing in faster and break the oxide layer.
Maybe finding a stand to hole the reflow gun above your work to free up a hand would help.
A good pcb clamp helps a lot too.