Collecting manually. I use 4 Haste Elixirs to collect (so far 2886 crystalariums and counting) so it takes like 7-8 minutes to harvest every 5 days.
And then there's 510 of Statues of Endless Fortune. They cost 1,000,000g each and have a 25% to generate 1 of the following: Iridium Bar, Gold Bar, Diamond or Omni Geode. Basically use the bars to craft Crystalariums and selling diamonds. Omni Geodes are used to trade Hay at the Desert Trader. They also generate gifts on Villager Birthdays so the gold they generate varies.
I kinda wish there was a higher priced statue for the end-endgame. Or just an expensive trader or something when there's so much gold generated. But so far they're being dumped into these statues.
Since each thunderstorm can generate up to 25 lightning strikes per storm (and it's not a guaranteed 25. I've tried placing 1000 lightning rods and you can still have as few as 3 battery packs created). The only way to increase battery packs is to use a Rain Totem during a thunderstorm to guarantee another thunderstorm the next day until an event happens. So you can have 1-2 week long thunderstorms to get as many battery packs as possible to make more crystalariums. And farms are required for this because of the Rain Totems, they require Truffle Oil (made from Truffles that pigs drop and then put into an Oil Press), Hardwood and Pine Tar (which requires a tapped pine tree).