would it be reasonable to copy a scroll from a scroll? i.e. -i don't have infestation prepared and rarely use it -i find an infestation scroll -i try and make more infestation scrolls from that scroll; for those o' so rare moments where infestation would be nice to have.

or do you think this would be to broken?

I don't think there are any rules specifically for creating a spell scroll by copying another spell scroll. The normal rules for crafting spell scrolls require that you have the spell in question prepared while crafting it; anything beyond that would have to be a house rule.

I think it would not be too big a deal to allow it, subject to all the other requirements for crafting spell scrolls (the time and money involved, and the required proficiencies).

since you can only have so many cantrips and given the simpler nature of cantrips (literally lesser spells you've memorized)
what would some good ground rules be for copying these scrolls?

Under both Tasha's optional rules in '14 and '24 standard rules a wizard can swap one cantrip per long rest. So without needing any homebrew could easily take one day to know Infestation and make your scrolls of it and then swap back in a cantrip you want more often.