I'm sorry to hear that your cancer has become castrate-resistant, and Keytruda isn't helping. I notice that Keutruda's makers don't list prostate cancer on their web page; are you getting it as part of a research trial? If so, then extra thanks for your participation.
https://www.keytruda.com/how-does-keytruda-work/
With 5 metastases detectable, you're right on the borderline: from my reading, some oncologists will still consider that "oligometastatic" and radiate each metastasis separately (assuming they're far enough apart), while others will consider it "polymetastatic" (high burden) and use a system-wide therapy like chemo or Pluvicto.

I hope the next step goes well for you.