You do realize the cost of replacing the logic board is about what you can find a 2017 model used!
How is the display and the case’s condition? Is the keyboard and the trackpad still in good shape? You’ll likely need to replace the battery as well as they do wear out! And lastly hows your SSD? It may need to be replaced as well.
So consider all of that in your cost analysis. If I were you I would look at getting a 2017 model new or used as it will be a better investment. Having a second system is not such a bad thing either!