In a long overdue update, I am happy to report that Sumo got his groove back. Since May especially, he just about returned to normal, his energy back up, able to jump up on beds and benches, eager to pursue bunnies, alert, wagging his tail in anticipation of my homemade food, eating on his own again, including regular kibble, on his initiative.
Sumo gets his CBD-oil only twice a day since May. I make a point to feed both of my pups a bit of raw tripe just about every day. It is supposedly so good for them, they tell me if I am in danger of forgetting, they like it. Buying the beef tripe dust raw in frozen little squares makes it rather economical, easy to use, with the stink far from unbearable. I transfer a few frozen squares into a closed container to thaw in the fridge and may later mix them with pumpkin puree or anything else suitable on hand. A bit of soft, thawed tripe is a great carrier for Sumo's oil and Isabella's thyroid medication.
I assume the vet was correct in diagnosing the mass in Sumo's belly as cancer. I can still feel the mass, but Sumo no longer suffers, so for now, we keep on going, taking one step at a time and hoping for the best.