I have a few mono albums, but not enough to make me get a mono-only cartridge.
My trusty Audiolab 8000PPA phono stage has a mono switch.
I also have a very basic stand-alone stereo/mono switch that I’ve used in the past. (An unpowered switch placed between phono stage and preamp.)
Some preamps/integrateds have mono settings, too.
Just curious how (if at all) you’re dealing with mono albums when using your SPP?