I'm so sorry that the news isn't better.
Yes, this could be cancer, but it still could be infection, considering how much pred has been given.
Btw, doxy might be helpful in cancers such as hemangiosarcoma and lymphoma and others (osteosarcoma, etc.)
Here is a "shoulda, coulda, woulda" thread. The dog should have made it (anaplasmosa). The striking thing is how well the dog did on doxy as the pred was weaned. Unfortunately, due to the vasculitis/edema, they kept going back to the pred. I don't know if it will be helpful or not, but the dramatic improvement is worth noting.
Doxy can cause a lot of stomach upset - I don't have problems with it here if I use the capsules. Some folks use probiotics, pepcid, and/or l-glutamine between doses.
Still wishing for the best for you guys :hugs: