In KaOS, Calamares is so configured that it will only take passwords that I think are too short. I don't recall the upper limit but it won't take 30 characters or so. In other distributions using Calamares, there's no such limitation. What is the reasoning behind this limitation, if any?

Others probably keep the Calamares defaults (not checking for minimum or maximum password length):
https://github.com/calamares/calamares/blob/calamares/src/modules/users/users.conf#L118
KaOS had a poll a few years ago about how to present the password checking (strict or not using password quality check and setting min & max lengths). Result of the poll was no strict checking, minimum length 8, max 25 ( so even less than the 30 you mention).
https://github.com/KaOSx/calamares/blob/master/src/modules/usersq/usersq.conf#L32

No-one has ever questioned or complained about these settings since. For one user, you can easily adjust, just edit /usr/share/calamares/modules/usersq.conf in live mode and set the length to -1 (or whatever max length you prefer).
If more now prefer different settings, a new poll will be needed.