Take a look at https://recapamac.com.au/ . The chap is no longer in the recapping business, but offers some helpful resources. Also consider replacing the PRAM battery – I’m not a vintage iMac expert but know that this has been an issue with other older Macs.
I can’t speak for cross-compilation, but I was a serious Mac developer on System 7 - MacOS 9 back in the day, and Metrowerks CodeWarrior was my tool of choice for C/C++ . A thread on 68KMLA from a few years ago has an extensive discussion of CodeWarrior: https://68kmla.org/bb/threads/metrowerks-codewarrior-best-version-and-updates.42338/
Bravo for keeping this great old software and hardware going (System 7 is perhaps my favorite OS of all time). Let us know what you build!
What the…? I feel old.