The Pixel 3 series was the last phone that Google offered the free unlimited photo storage at original quality. Even the Pixel 4 series does not offer that benefit.
You have two choices: 1) save photos in high quality or 2) continue using unlimited storage and eventually have to buy more storage from Google.
Since the "High Resolution" option saves photos up to 16MP and the camera in the 3a is a 12MP camera, you should be able to use this option without any loss of photo quality. Videos will only be saved at 1080p, but honestly that is fine for archive purposes. If you need 4k for editing purposes, you are likely pulling that footage off the phone and not using the Google Photo version to edit with.
So unless there is some real reason you need videos archived at 4k, the "High Quality" option is completely fine with the 3a series.