I never owned a Commodore64, though I did fix a large number of them because a computer store in Amsterdam had so many DOAs that there was good money to be made turning five broken C64s in to four working ones and a couple of spare bits.

The machine I did most of my 6502 stuff on was a BBC Micro, which had a basic assembler built in, so that really helped but the KIM-1 which I used before that was so sparse that even though an assembler existed I would have first had to expand the memory (big $back then). So memorize opcodes and use the handy 'opcode card' that you could get for those CPUs back in the day. No big deal, though I was really happy once I moved on to automating that part of the process. Especially branch calculation is tricky because if a branch is on a boundary you could fix it easily in something with macros but by hand it is hell because if the branch can't reach the target you need to invert the branch and then use a JMP, which requires more space, which may cause other branches to no longer reach their targets and so on.