extra backup HD, using TM

1. Yes. Use GUID and Mac OS Extended (Journaled, Encrypted).
2. Time Machine cannot use that format.
3. Yes. A single backup drive can be used to back up multiple Macs.

Beware that excluding system files will preclude the ability to use that backup to completely restore that Mac should it become necessary. Excluding system files is not worth the small amount of space it would save.

Hey there! Good questions!

Agreed it’s best to format a drive as encrypted during the initial erase/formatting of the device.

I don’t believe APFS is yet available for external drive formats yet, (unless that’s changed), but HFS (Mac OS Extended Journaled), should work just fine.

I’d only use Master Boot Record if you plan on also using it with Windows and Mac both. (And formatted as ExFAT),

And yes, although the backup drive is encrypted you can access the data on another Mac, (provided the OS or hardware on the Backup isn’t too old), you’d just need the encryption password in order to get to it, if you forget it, you’re going to have to erase it again,

You can also backup more than one Mac at a time, it’s best to make sure the Macs have different names to differentiate them on the external drive. (You can name your Mac in System Preferences > Sharing).

And if you still wanted to move the data from your encrypted data to another Mac selectively, you could Do that using Finder, but opening the external drive, (Which would require the same password), opening the Backups.backup folder, and opening the date of the backed up data you want to move.

Hope that helps!

https://support.apple.com/en-in/guide/mac-help/mh21241/mac

1. It's faster to encrypt the drive before use. Drive should use the GUID partition scheme and formatted APFS (SSDs) or JHFS+ (HDDs.)
2. An encrypted drive can be used on any Mac, if the version of macOS is compatible with encrypted drives and you know the password.
3. You can use any backup utility for your backups including Time Machine. However, I would recommend one of your backups should be a clone of the startup volume. For this chore I would recommend Carbon Copy Cloner of SuperDuper!.