Timeshift snapshots

I try to take a snapshot after every update to a USB thumbdrive. Recently I received the message that there was not enough space and without further thought I went in and deleted several snapshots starting from the oldest. On reflection I relaised that Timeshift moves on to incremental snapshot so I may have deleted the most important base for the records.
How does one handle a situation like this? Always retain the oldest snapshot or delete the lot ans start from scratch again?

Never delete something manually from incremental or deduplicated backups. Always delete it using the respective backup software.

Usually, there is a prune option, that automatically prunes old archives. Never used Timeshift, though.

I use Timeshift all the time. It is a very good feature of Linux Mint. And it has saved my a** more then once. And @Akito you are right about having a purge (prune) option, Timeshift has both a manual and auto purge options. Auto purge is base upon the number of Timeshifts you want to keep. @KenWA, I would use the manual option within Timeshift to delete snapshots next time if you didn’t use it this time.
But to answer your question @KenWA, I can only give my opinion from using Timeshift b/c I am no expert. I like to think of snapshots like a point in time or if you will a picture of your system from the last snapshot. So once a snapshot (a Timeshift) is deleted, you can not go back to that point in time.
I don’t believe you need to worry if your system is running OK. If your system is running fine, your last Timeshift (snapshot) can be your new base. Does the Timeshifts files you deleted still show up in the Timeshift index? Do you take regular backups?

This may be applicable in your situation:

I use BackInTime under Ubuntu 20.04. It also performs incremental back-ups. According to the help documents, it creates hard links during the back-up. No file will be deleted if anything is still hard linked to it. So you can delete the oldest back-up or any of the intermediate ones and those remaining are still complete.

Timeshift may work in a similar manner.