Advice please - Euthanasia for an elderly rabbit and donating their body?

As-salamualaikum everyone, I have an elderly giant rabbit who’s been diagnosed with heart failure, arthritis, and a very aggressive soft tissue sarcoma on his hind leg. When the cancer was first found two months ago the vet said it was terminal and pencilled in a euthanasia appointment later that week if he began to struggle to move because of where the tumour is. Alhamdulillah, with medicines, antibiotics and regular bandage care from us, he’s lived two months longer than expected. Lately he’s showing more trouble with that leg - the toes look paralysed, he sometimes loses balance when he hops, and he has a hard time scratching himself. He just isn’t as active and the dressing changes are becoming really distressing for him. The vet’s point was that it can be kinder to choose euthanasia while an animal still has some quality of life (he can still eat, drink and take meds) rather than wait until he’s clearly suffering, unable to eat and in severe pain. The explanation was that animals live in the moment and won’t understand a future recovery, so avoiding prolonged suffering can be merciful. I have two questions and would really appreciate guidance: 1) Given this is a terminal illness and will very likely get worse, is it permissible (halal) to opt for euthanasia now to prevent further suffering, or should we only consider it once he’s visibly suffering a lot? I want to do what’s best for him and what’s right in Islam. 2) What do people think about donating a pet’s body to science? There doesn’t seem to be much research on this type of cancer in rabbits and I thought donating might help future animals. On the other hand I’ve heard remains are often cremated after research and that makes me hesitant for religious reasons. Any advice, personal experiences, or Islamic perspectives would be really helpful. Jazakum Allahu khairan for your time and duas 🙏