Rear Camera and Flash is not working. Lagging front camera.

Things may have changed with the release of iOS 18 and the Repair Assistant, but outside of that, Apple has made sure that logic board replacements are no longer a viable solution. The newer the iPhone, the more parts are paired to the motherboard. Starting with your phone, the following parts are all paired and will cause loss of function and/or scare tactic warning messages.

• Battery
• Display
• Camera
• Back Glass
• iPhone Rear or Mid System

Unless the Repair Assistant will let you re-pair all of those parts, you would have to buy a motherboard that includes all of those paired parts in order to have a fully functional phone. Given that list, you might as well just buy another phone since you're purchasing most of the important components anyway. It's possible that iOS 18 will let you pair the parts again, but I don't have enough information to say if that's possible or not.

That being said, you did ask about a guide for replacing the motherboard, and as you've already found out, iFixit unfortunately doesn't have one. However, there is a Spanish site called Nadie Me Llama Gallina that does have what you're looking for. It is, of course, written in Spanish, but used with the Chrome browser's translation feature it's quite usable and their guides are well done.

Manuals / Apple iPhone 14 Plus / Motherboard - Nobody Calls Me Chicken

Good luck, and if you get anywhere with the Repair Assistant we'd love to hear what you find out, so be sure to come on back and let us know how it all turns out for you.

Oh, and you'll lose Face ID for sure if you swap out the motherboard without all of the parts associated with it, including the IR camera, the dot projector and the flood illuminator.