Late 2019 Macbook Air with Intel Core i5 8GB RAM 128 SSD
No, the processor is soldered. I highly doubt buying a logic board is a good choice. You can't buy it from Apple, so you're stuck with third party options. It's also fairly likely that a working logic board that you want is going to be fairly expensive, maybe even more expensive than just buying a new laptop. 

Your best value is sell this mac - perhap helping someone get it set up so they are happy paying full price and then order the gear you want paying the delta in what your sells for and what the new costs.