Resolved.
For posterity, the solution is that you cannot use your master account password to sign into your CalDAV on iOS/macOS, youâve instead got to create a new device/session and use those credentials. Here are the steps for thegood.cloud:
Navigate to Settings > Security > scroll down to Sessions & Devices > type in a new app name (e.g., âiOS/macOSâ), enter your master PW, then copy down your newly generated password > go to your Mac/iOS device and add a CalDAV account > Advanced input > enter your email, password, the server address (use08.thegood.cloud), server path (/remote.php/dav/principals/users/YOUR_USERNAME/), port 443, SSL on > Voila!
Hopefully this helps.