Big project alright. Excavate well past the line of the pool, and down no less than 6" below top of concrete of the garage wall.... the lower the better until retainage on the other side becomes the issue.
Please let me counsel against railroad ties, especially the used ones. They fail too soon, are not reliable even when delivered, and never look nice. The method of choice these days is the keystone stacking block, the biggest (most massive) you can wrestle with, and especially get the foundation course right. Proximity of that driveway adds migrating load, so build that wall high and tight to resist. Good luck. Starting right is vital in your case since everything you spend $$ on after that depends on it.