Would I be able to swap a 2018 motherboard, the same size, to my 2019?

Sure you can swap out your logic board for a better one from within the same series. For referece these are your choices: MacBookPro15,2. You can’t install the other boards into your system.

But be careful! You need to swap out the power button assembly with the logic board that’s mated to it. Here’s a good source 13” MacBook Pro 13 Logic Boards if you have a two USB-C port system you ‘ll need to focus in on these boards.

Sadly, you might not find a logic board that offers the amount of RAM and storage you are looking for, you may need to look for a used system to just use or if the case is in rough shape swap out it’s logic board with yours.

Here’s the guide MacBook Pro 13" Touch Bar 2019 Logic board Replacement