"cron" processes spawning and piling up, but no crontab
So I found a relevant answer here:

Cron job keeps running, not in any crontabs

I don't think this question should be removed because this seems to be relevant to Catalina, even though it seems to be user error.

Explanation:

The root crontab had been edited at some point, with cron jobs that I created 6 years ago on my computer (I may have done this before learning that was a bad idea). This crontab had two lines that each referenced a script that was supposed to run every minute. Neither one of these referenced existing locations on my drive, and for whatever reason cron wouldn't exit after attempting to run them.

This problem never manifested before upgrading to Catalina, but I don't know why that is - either cron was running every minute for the last 6 years and the Catalina update semi-broke cron, or something got copied to the root cron file. Probably the former problem, which would be a PIBCAK scenario combined with typical Catalina issues.

The solution was to edit the root-owned crontab to remove the two offending lines, and boom, the problem is gone!

I created a new user crontab for testing, and this leaves one persistent cron job (with parent = launchd), but the rest of the cron jobs that I run will exit predictably and not suck up resources. Cron itself doesn't seem to be broken, but root-run cron jobs may be problematic. (EDIT: I deleted the root crontab and now activity monitor doesn't show any cron processes running - I'm mystified as to why this happened with only certain user accounts, since none of this happened at the user level)