Yaarel
🇮🇱 🇺🇦 He-Mage
The twohanders were designed for half-swording, as their elaborate hilts make clear. At the same time, half-swording does less damage. The damage done by the full momentum of a twohander swing is much more (and much more dramatic) than pushing a half-swording stab.I'm going to say this one more time: Half-swording was how greatswords were designed to be used. It was not a sub-optimal situation for them. What you're proposing here is like writing rules for shotguns that make them sniper weapons.
D&D is so close. If it went with a meter/yard square grid, instead of a 5-foot square, it would be straightforward to track the combat distances. And the advantages of the reach of different weapons: adjacent hand-to-hand range (including grappling and daggers), sword range, and polearm range (including spear and greatsword), plus occasionally a pike range.That's how it works in some games with finer resolution on the combat map. For D&D and its 5-foot squares, it doesn't really track well.
D&D can probably approximate this by means of conditions. For example. Perhaps a ‘reach’ weapon or a ‘polearm’ weapon can allow a character to attack without becoming ‘engaged’, thus can move away without provoking opportunity attacks, thus has no need to spend an action to ‘disengage’. Probably all long weapons, including spears and greatswords, should both allow attacks at reach and make hostiles unable to ‘engage’ thus unable to attack with nonreach weapons.