External hard drive with old system unreadable

Blessing the system should have no impact on being able to access the data on the drive. It just adds a couple of hidden files to the drive to indicate the drive has been blessed and "bootable".

On macOS the file/folder permissions are based on the UserID of your macOS user accounts, the user name has no bearing on this (except perhaps for extended ACL permissions -- I haven't done much with ACLs). If you have only a single user account on both drives (and never added or deleted any from either one), then you should be able to access the data on the other drive for a matching user folder with no problems since the userID should be the same.

In the past I have been able to copy the whole folder inside a macOS user account (Desktop, Documents, Downloads, etc.) which are locked due to incompatible permissions since macOS will prompt you for your current admin password when attempting to copy the locked folders. Of course this will copy all of the items within that top level folder (Desktop, Documents, etc.). Maybe later versions of macOS prevent this now. You can use a third party app such as Carbon Copy Cloner (CCC) to access all the items on the old drive and pick which files/folders you want to copy to the new system. CCC will prompt you for your admin password.

I do not recommend keeping this old drive and accessing the data when needed as it is an awkward and dangerous option (I've done this before when dual booting and it gets messy quickly). It is fine to keep the old drive if you may need to boot it again, but since you've been trying to manually "bless" it, then it probably will no longer boot. If the drive is that old, then I would be concerned about a drive failure at some point if the drive is not already worn out or failing (you may not notice the failure at first). Plus if you want to boot this old drive again, then changing permissions will cause problems. If, however, you will not be booting this old drive again, then changing the permissions won't hurt anything in order to access the data. Just pull the data off the drive and store it in a manner where your other Macs/devices can access it. Sharing an old OS drive is just asking for trouble in so many ways.

I'm a bit concerned that you need data from this old drive especially needing access from multiple computers. It implies you don't have any backups which is very dangerous if you have any important data. Drives fail and accidents can happen which can destroy all data on a drive many times with no hope of recovery even when using an expensive professional data recovery service. If you need to share data across multiple devices, then you have multiple options available such as iCloud syncing between your devices (or some other cloud file syncing service if you need to use non-Apple devices as well), or a network storage device (NAS), or an external USB drive that has been completely erased so it only contains your data files. No matter which option you choose here, you still need to back up this data since cloud file syncing services are not backups and a single drive with shared data is a huge risk if you value the data.

You should always have frequent and regular backups of your computer(s) and all external media (including the cloud) which contains important & unique data.

You could Apply/Add rights to the Users Home folder on that drive & "Apply to enclosed items" .

You can uncheck put drives to sleep when possible in Sys Prefs>Energy Saver.

No, you can add a user to the Home folder... tiny plus icon in the Get Info window.