Would it be possible to remove science packs spoiling but preserve the resulting science pack's health so the puzzle of producing them with freshest possible ingredients would remain?

Removing spoil time completely also removes freshness.

As a workaround, you can set the spoilage rate to 0.000001 in the mod settings. That basically multiplies all spoil times by a million, so the science packs still have freshness, but they take several years to actually spoil.

Freshness will still go down over time, just extremely slowly.

Edit: Make sure you have the latest version of the mod. I fixed a bug where extremely low spoilage rates could create values that were too large for Factorio to handle properly, which ended in a crash. Should now be fixed.

Got it, thanks for the response