I'm having trouble fully understanding your environment based on your description but it sounds like a point-to-point setup might be a good solution. You would need two devices at around $100 each plus a switch and access point on the other side.

They are meant for line of site which is why the 50' courtyard made it pop into mind. However these bad boys are designed to go miles so at that short of a distance a wall or two shouldn't be that detrimental, depending on what the walls are made out of. You'll defiantly get better than the current reduction from 300Mbps to 20Mpbs down you have and it will be consistent.