Does it track milestones for the current playthrough only....?

For instance keep a history of previous playthroughs and report, "Previously you got green science 45 minutes, This time you took one hour." Or "New record for completing Yellow science. 5:35, you best before was 6:24."

Yes, there's no historical data from previous games, for 2 reasons:

The first is that mods can't access anything from outside the current game, so trying to implement that would result in a weird user experience. I'd have to allow exporting your records as a string and then players would have to save that string somewhere and paste it in every new game, and not forget to update the string when they get new records. Ew.

The second is that it's a little out of scope of what I want for this mod. For more serious speedrunning, it's probably best to use out-of-game generic splitter programs like Livesplit.