Checkpoint64 has a preset for Stellaris: it already knows where the save folder is, backs it up automatically and keeps every version. Everything this site has on Stellaris:
Stellaris
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Save file location
Windows:
%USERPROFILE%\Documents\Paradox Interactive\Stellaris\save games
macOS:~/Documents/Paradox Interactive/Stellaris/save games
Linux:~/.local/share/Paradox Interactive/Stellaris/save games -
Backup guide
Ironman keeps one save that overwrites itself, and a DLC or mod update mid-campaign can leave a 400-year game unloadable. Checkpoint64 keeps every version of your save games folder, so a lost empire is one click from undone. Free download.