Time machine unable to go back in time

I understand you are unable to retrieve any TM backups other than the most recent one. There are a number of possible reasons for that, ranging from the mundane (insufficient space for more than just one) to technologically challenging problems.

Review If you can't back up or restore your Mac using Time Machine - Apple Support for some remedial suggestions and please write back if you need help.

Sometimes, it becomes necessary to "reconnect" to the backup drive. This can occur if your backup drive is on a network (TC or AEBS) and there is a change to that network, such as a momentary power interruption for example. The following technique describes the procedure. It's harmless and will not affect any existing backups.

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The following technique can be used to "reconnect" to the backup disk. It will not affect the existing backup history in any way.

Open Time Machine's Preferences and de-select the Back Up Automatically checkbox. Make sure no backups are in progress, otherwise you should wait for it to finish or terminate.

• If the backup disk is directly connected to the Mac, disconnect it: drag its icon to the Trash, wait for it to disappear from the Desktop, and then physically disconnect it from the Mac.
• If its icon does not appear on the Desktop, select External Disks in the Finder menu > Preferences... > General. You can de-select it later.

Then, click the "Select Disk..." button, select the backup disk, then Remove Disk, then Stop Using This Disk.

• If you designated more than one backup destination, the "Select Disk..." button is replaced by an "Add or Remove Backup Disk..." option. You may have to scroll through the list of backup disks for that option to become visible.

This will not affect any existing backups.

• If the Mac is directly connected to your Mac, reconnect the backup disk, power it on if required, and wait for its icon to appear on the Desktop.

Then, Select Disk... again in Time Machine's Preferences. Re-select the same one under Available Disks, then Use Disk.

• If the backup is on a network (TC or AEBS), you will be prompted for the password you provided for that disk in AirPort Utility.

"Waiting to complete first backup" will appear, which means it won't do anything until the next scheduled backup. Despite the implication of that message, it does not mean it needs to create a completely new backup, as if it's creating a brand new one. Your existing backups will still be available.

• If you don't want to wait that long, select Back Up Now.

A Notification may appear if your source volume is encrypted and the backup volume is not. That's normal.

Re-select Back Up Automatically. The "Preparing Backup..." status message will remain for a long time. Eventually, "Backing up xxx of yyy" will appear, but those values will not be accurate, nor will the "Estimated time remaining" in Time Machine's Preferences. Just ignore it. It may take a few hours even if your Mac is not allowed to sleep. Time Machine will also pause or become slow if you use your Mac for tasks it considers a higher priority.

Is it possible that those older backups predated upgrading macOS?

Still, they should be available. You may be able to view them by holding an Option key while selecting the TM icon in the Mac's menu bar, and selecting "Browse Other Backup Disks..."

There are some circumstances in which Time Machine will segregate older backups, leaving them untouched. Changing a Mac's name (for example) will cause TM to conclude you are backing up a brand new Mac. That can result in insufficient backup disk space, because those older backups will never be deleted. The solution is to erase the backup drive and start a new set.

Two backup drives are definitely a good idea. Time Machine will alternate its backups between them. It will do the same for as many additional backup drives as you wish to use — it cycles through them sequentially.

Is that an inherent problem with Catalina?

I don't think so, but it has been a while since I used Catalina and it's been almost as long since I upgraded to Big Sur and finally Monterey.

Should I update to Monterey?

I would, yes. Especially since you already have a Time Machine backup (and now, at least two of them).

I think the bottom line is don't expect TM to retain a long backup history. If you have had no need to restore a TM backup for a year or so, my recommendation is to completely erase that backup drive. Of course you should not do that unless you already have another backup drive, which you do. You can erase that one a year later, and so forth. That's essentially what I do. After all, why do I want backups from 2017? I don't.

It doesn't take long for TM to build a backup history consisting of many millions of tiny little files, and it just gets unwieldy.