Hemangio is a hopeless diagnosis. Too many of us have lost dogs to it. It might as well be called "Shepherd Cancer." The world of GSD owners seems to consist of two groups -- those who've already lost one (or several) dogs to this wicked cancer, and those who yet will.
You probably don't have long. Even if you don't choose when, it will still happen sooner than you'll ever be ready for -- this evil cancer waits for no one. If the dog is experiencing a major internal bleed, it might already be happening in slow motion. Or the blood may reabsorb, and you'll get a few more days that seem better -- and then another worse crash.
When you're ready, my advice is to try to locate a mobile vet who will come to your home, if your regular vet won't do that. Give your beloved friend a gentle, peaceful passage from this world, surrounded by family, at home. This way, the dog can be at peace, safe at home -- not in a stressful clinical environment. (You'll also be in an emotionally safer space for the aftermath -- sobbing in a vet hospital, with strangers staring at you as you pay the bill, somehow makes the heart-wrenchingly bad experience even worse.)
My friends who've tried to fight Hemangio deeply regret having done so -- the spleen removal, chemo, and frequent vet trips made the ending less peaceful, and it still comes. One I know passed on the operating table during the spleen removal -- her owner didn't even get to say goodbye. The distraught owner was bereft. Those are not good endings.
Choose a final passage worthy of your dog's love for you. Be gentle with yourself, too, as this is a hard, hard thing to do.
One suggestion: on the last day, lie down with your dog and tell him the story of his life, from your earliest memories together, recounting all the amazing things you did together, and all the joyful memories. I did this with my last one, and he listened so intently as I talked for hours, remembering the things we'd done together. Those were very precious final hours.