Just a thought, have the C64 version use writes to $d030 control which cache mode to use, somewhat like the C128, SuperCPU and Ultimate64 kits do to switch between 1 MHz and whatever speed they run at?

(ref: comments a few lines down in this thread https://www.lemon64.com/forum/viewtopic.php?t=75567&sid=f14af11e92b12b174d17ca269f6ef23a )