I have been in this exact position, except it was maybe worse.
Family member could not handle the pit bull they adopted years prior - kids, divorce, changing living situations. Rescue they got the dog from would not take the dog back. They kept the dog in a crate in the basement for nearly his entire adult life. Dog was 3-4 years old. They knew I was not a permanent home for the dog, but a stepping stone.
The dog came to me very snippy about toys, extreme resource guarding, WAY too much energy, human aggressive at the vet (would take out anyone and anything the second he walked in the door, and he was NOT kidding around), and to top it all off he had some serious physical issues that prevented me from working his energy level down without making the following day the worst day of his life. The vet aggression was bleeding over into “anything that made him even slightly nervous”, towards anyone.
I tried for almost a year to fix that dog. Lots of things improved - he was far less snippy about toys, had much better impulse control. But the vet aggression, high energy, and physical issues remained.
I warned the family, what I was trying, and what options were on the table. No one volunteered to take him back. No one even said “yes I’ll take him but give me a couple months to get my XYZ in order”. It was “no, he’s your problem now.” I called hundreds of rescues (not a lie). I was willing to pay his transport, anything, everything. No takers.
I euthanized him after a year and 5 months of effort finding him a good place to be.
And now? The family posts about how much they “miss Opie” and talk behind my back about how “I killed him.”
All that to say - OP, it’s hard. But your old dog has tenure, period. If a solution can’t be found in short order, something has got to give. You have a heart of gold, and you’ve tried everything. He’s a beautiful dog, with some big issues to get over.