alt Hacker NewsAnother useful tool was a debugging cartridge.
My C64 came with a Super Snapshot. It had a button on the cartridge that would freeze the running program and provide options like editing the memory, modify machine code, persist current state to disk, etc.
I would type programs in BASIC and then examine the machine code that it got compiled to. I learned a lot more than I would have without the cartridge.
Young me is envious. I downloaded or typed in, I forget which, some monitor program[0] that I used a lot, but loading it was far more manual than what you described and was basically impossible to load while another machine code (i.e. non-BASIC) program was running.
I still learned so much from typing short programs into it. I talked my parents into buying me an assembler so that I could make changes without re-typing a whole program.
[0] https://www.c64-wiki.com/wiki/Machine_Code_Monitor