My left trigger is loose and the spring is making a squeaky noise.

It's brand new? Or just new to you? If it's brand new, I would say that's cause for swapping it out for a new one. Configuring a new product is one thing, but repairing it is another.

If it's just new to you, I would have a look at the trigger yourself! Steam Decks are pretty friendly to get into, you'll just need a Torx (T6) screwdriver if you don't already have one to get the rear cover off. Getting at the trigger mechanisms doesn't even require removing any other parts just disconnecting the battery.