I can fully relate to this sad news.
My Dal, AJ, passed on Sept. 24th. Just 10 days earlier, I took him to the vet as I noticed he was being a picky eater - only eating his absolute favorite items, but leaving the rest - and drinking more. This was odd for him.
The vet determined his spleen had a large mass on it. I opted to have the surgery to remove the spleen and the mass.
AJ seemed to recover well from that initially, but then a secondary tumor was found on his spine. It is unknown if the two are related (rare for a tumor on the spleen to go to the spine), but there were also spots on his liver. The tumor on his spine was inoperable and apparently a heartbeat from doing serious damage (it already caused some limitation in his mobility in his front legs).
But a steroid helped him regain quite a bit of mobility and he seemed to healing. I knew his time was limited, but felt I had weeks or even months. Sadly, the day after I saw the most recovery and significant improvement, he passed. A mere 10 days from diagnosis to death.
Had I done nothing, the spleen would have ruptured as it did with your beautiful pet. Sadly, I still lost him anyway. Sigh.
I wish us both nothing but healing hugs as we both recover from our shocking losses.