I have a shiny Pidgeot in my let’s go Pikachu (one of my only two shinies I got outside of ultra wormholes or trading, and I got it without a shiny charm) but I normally play usum so I want to get it into usum. Is that possible?
Unfortunately, it is not possible. Pokémon Home allows only one way passage from 3DS games, so Pokémon cannot be transferred from Let's Go to Home to Ultra Sun/Moon.
Source: https://www.eurogamer.net/articles/pokemon-home-guide-transfer-from-switch-3ds-go-bank-7017