Might I ask what terminal program you're using? I PIP'd enough code over the serial port to assemble a basic Xmodem client, but it's unreliable in use.
I came in at the end of the CP/M era, with an Amstrad PCW9512. I loved it. Sadly a long-ago ex-G/F gave it away. :'(
I have 2 replacements, mind you. But CP/M is kinda dry. It's not much fun to play with. I am at a bit of a loss as to the new enthusiasm for it today...
But CP/M...man, I wish the Kaypro ran something more interesting, because it deserves better.
There are or were some alternative Z80 OSes: SymbOS, Cromix, Uzi/Fuzix, S-OS, DR's own MP/M... Not sure if any would run on a Kaypro without significant expansion, though.
Some of the early-1980s Forth environments were impressive. I read of a replacement ROM chip for the Sinclair ZX-81 which could handle multitasking and windowing.