This question may seem like the Pokémon GO equivalent of a First World Problem. I get that. Some players do not have the time, money, or access to raid frequently enough to wonder what to do with their excess Legendary Pokémon. However, other trainers are clogging their storage with hundreds if not thousands of Legendaries caught in raids. So… what does one do with all of those Legendary Pokémon?
If you raid frequently, meaning multiple times per day or even just someone who goes wild during Raid Hour, you're going to end the week with potentially dozens more Legendaries than you began with. Even though Niantic has increased the storage cap multiple times, there is a limit to what you're going to want to keep.
Now, the most basic advice is to, of course, trade with a Best Friend in Pokémon GO. If you're Best and you both have the Legendary unlocked, the Stardust cost is minimal and the hope of getting a Lucky Pokémon is worth the planning to make this happen.
If you are an avid raider, the number of Legendaries you're pulling in weekly greatly surpasses the number of trades you're going to be able to do. Special Trades are allowed once per day, with events allowing just a few more happening very far and few between. During a normal year of Pokémon GO, assuming there are two events that allow for five Special Trades per day, you're still getting under 365 Legendary swaps. In addition to that, Niantic recycles Legendaries very frequently. Heatran was featured for a month in January 2020, again for three weeks in late summer 2020, and will probably be back any week now.
How many Lucky Heatrans do you need, probably Brandon Tan?
Here's my advice. Swap weather-boosted Legendaries with Special Trades at your convenience, and then horde the rest of your extra Legendaries until there is a Spotlight Hour with the bonus of double transfer Candy. And then? Bye-bye, Articuno. It may seem sad to let a Legendary go, but the truth is that Pokémon GO encourages grinding… and if you are a player reading this article looking to see if you should do this… well, you're definitely grinding raids.
As one final tip, I'd encourage you to keep three or four extra of each Legendary, just in case you run into someone who needs one. There's no better feeling, and no better use of an immense amount of Stardust, than giving a newer player who missed an older Legendary one of your doubles.