Replacing the whole motherboard is a terrible way to upgrade RAM.

Technically yes, but practically no.

You can swap the entire motherboard for one with more soldered RAM, but that’s not a meaningful upgrade. It’s basically buying a new laptop sans screen and keyboard. The cost of a compatible board often approaches or exceeds what you’d pay for a whole new machine. And if you’re not comfortable with a deep teardown, you’re paying a repair shop to do it.

Unless you have a specific reason to keep the chassis (rare), just buy the laptop with the RAM you need from the start. Soldered RAM is a trap. Don’t dig yourself out by swapping the whole floor.