I went through the same process re-reading 1e, OSRIC, 2e, looking at LL and then getting OSE + Advanced. In the end I just couldn't play with the old school systems. I'm currently enjoying Castles & Crusades.
C&C gives you the 1e vibe but with the SIEGE system that is not 3e or OGL. The current 8th printing of the Keepers Guide, with the AD&D homage covers, is full of optional rules to customize your campaign. You can play beyond level 14.
I finally found a game that feeds my nostalgia for old AD&D. The search is over. It is well supported with many adventures and campaigns series.