don't think you understand how eHP works. as kayfabe mentioned, eHP = hp + def. it was used primarily for handling golems and their high attack stat. some of the golems hit 2.5k, 4k, and 5.2k, hence eHP was calculated to see which units were able to survive a single hit from those golems. def is less of an issue due to enemy attack being very slow such that a healer can get more than 3 heals in between each attack. Leeanne actually has really low ehp, barely able to survive a single 2.5k attack, but her defense is high for a silver and usually does better than garrett.
as for rest of topic, not going to read in-depth, but garrett is most likely not the best choice of unit to focus on leveling. Leeanne usually suffices for most content and Garrett usually isn't good enough for anything that leeanne can't tank.