Thank you everyone for your responses! It's been bugging me all day and there weren't really clear answers online that I could find. I think from now on then I'll just go w/ the 20 pack since they're usually cheaper and then I can hopefully have enough to last me a couple cycles.
Even though I was really surprised with how early it was, I have to say it was ridiculously exciting to get the smiley face haha. So far I've just been BDing around the time I thought my fertile window was based on calendars and CM, but if this cycle is any indication, it looks like I've been pretty off - it's exciting to finally have an idea!
I've also been using SMU at work to test, but after hearing that the FMU has been fine in your experience, I think I might do that - it'd be so much easier than trying to discreetly sneak off at work with the holder and stick haha. So thank you for that info too - they say any time works but it's good to hear someone actually have experience with it.