I have played Waterloo 4 times, with Napoleons Battles, Republique and twice with Volley and Bayonet.
V&B gave by far the most satisfying feeling of commanding troops in a large battle.
BUT, none of those are 1:20 so won't suit the unit sizes you want. You are looking at either "General de Brigade" or "Republic to Empire", the latter probably coping better with that many units per side.
Either way that is a major undertaking.