"I pulled out my older machine and was able to access the older backups that way. So there’s some step missing that doesn’t allow you to access backups from older machines/OS. Any info or feedback welcome. Thanks."
OK, trying to assess the situation.
You have an older Mac, with data you'd like to access on a newer Mac (but the backup for the older one won't work on the newer one)...
Is this assessment correct?
If so, I have an answer that will work.
PRINT THIS OUT and follow every step.
What you'll need:
1. an external drive large enough to hold all the data on your OLD Mac
2. the app named "SuperDuper", which you can get by clicking this link:
What to do next:
You didn't tell us what version of the OS is running on the older Mac.
a. get the old Mac up and running
b. connect the external drive
c. open disk utility and erase the drive to HFS+ (Mac OS extended with journaling enabled, GUID partition format). If it's a newer version of the OS that needs APFS, use APFS instead.
d. when done, quit disk utility and open SuperDuper
e. SD is VERY easy to understand and use. Use SD to "clone" the internal drive of the OLD Mac to the external drive
f. SD will create "a clone" of the internal drive to the external. It will be mountable "in the finder" like any other drive you have.
And then...
1. take the cloned backup you just made on the old Mac, and connect it to the NEWER Mac.
2. Let the drive icon mount on the desktop, but DO NOT OPEN IT YET
3. Click ONE TIME on the drive icon to select it
4. Bring up the "get info" box (you can type command-i)
5. At the bottom of get info, click the lock and enter the password you use for the NEW Mac (not the old one)
6. Put a check into "ignore ownership on this volume" (sharing and permissions)
7. Close get info.
Now you can copy stuff from the external drive to the new Mac, and what you copy will "fall under the ownership" of your new account on the new Mac.
You could also use the cloned backup with setup assistant on a brand-new Mac for initial setup.
Good luck.