The buses are regular and easy and there is luggage storage at the blue lagoon (not on the bus ime).

You can buy bus tickets in advance but if you want to go earlier or later they will let you, assuming the bus isn’t sold out. Blue lagoon also sells tickets for different entry times (though you can stay as long as you want) and those can sell out (imo the cheapest ticket is fine).

If you really want, there are taxi services which will take you to BL, wait 90 mins and then take you to the airport but they are expensive

(Also note - Keflavik airport is about an hour drive outside of Reykjavik - Blue Lagoon is very close to KEF).

Take the FlyBus. They have a good setup to take people to the airport via the Blue Lagoon for afternoon/evening flights out. They'll even hold your luggage on the bus for you while you're at BL. The buses are very organized and regular. Easiest thing to do is just go talk to the FlyBus people at the airport (outside customs/immigration in the public part of the airport) once you land to get schedules/advice. They also have offices in downtown Reykjavik. Don't overthink it - you will have no problem sorting it out once you're there. Enjoy!

You can book bus service from downtown and to the airport when you book your blue lagoon tickets.

I've tentatively scheduled a visit to the Blue Lagoon on the last day, before the 5 PM flight back to the US. Can anyone recommend the best way to get from Reykjavik to the lagoon and then onward to the airport? Is there a transportation service that makes this easy? Or just take a taxi to the Blue Lagoon? And then another one to the airport? Or are buses very regular and it's overthinking the situation? Anyone done this before who can advise on what to expect? Thank you!