Dog with b cell lymphoma

My 12 year old yorkie did CHOP chemo in August 2024 and completed it in December 2024. His cancer came back January 30, 2025 so he began CCNU chemo in February 6, 2025. We found out it wasn't working so he started mitoxantrone chemo on April 3, 2025. On April 24 we found out it wasn't working so he just started a different chemo I can't remember the name but it is a drug from the chop protocol that begins with a d and I can't find the paperwork right now. If this doesn't work I'm wondering if anyone has gone through this with their dog and what the next steps are. Should I get a second opinion with a different oncologist? He has never shown any symptoms of his cancer except swollen lymph nodes and he had bruising on his tummy when I first took him to the ER and that's how I found out he had cancer. He eats , drinks and plays normally the only difference I've noticed is when his lymph nodes are swollen he gets very protective of me. Thanks for any info regarding what it was like for your dog's cancer journey.