Both joy-cons had drifting issues, and the left one was loose because the lock was worn down. Compressed air solved the drifting for a while, but eventually it always returned. I replaced both the analog sticks and the locks on both joy-cons. I ended up utterly destroying the left joy-con's battery connector when putting it back, so I had to replace the battery too. Lesson learned - don't use sharp metal tweezers to press in a soft plastic connector upside-down.
I replaced the plastic locks with metal ones. They have a nice, satisfying click, but surprisingly aren't more durable than the plastic ones. Only a day after replacing the left joy-con's battery, I dropped my Switch and the left joy-con went loose again. When I inspected the lock, it had chipped in the exact same way the plastic one did. Thankfully the kit I bought had two sets of locks for each joy-con, so I was able to replace it again.
The new analog sticks are completely fine - in fact, I think their click action is more satisfying than the original's. They were weirdly calibrated out of the box, but they work perfectly with recalibration.