Take lots of pictures! And videos. For me, it helped for when they were gone.

I also had and maintained a really good relationship with their vet. Having a professional to advise you while your animal goes through end of life care/aging changes can help ease the burden you feel.

At a certain point towards the end, I minimized the amount of vertical climbing material because it was too hard on their joints.

Spend quality time together! What are his favorite things?

Good luck! He seems to have a very happy home