I love 5e.
Longest small party combat I was involved in with 5e took 3 sessions at 5 hours each for a party of six against multiple baddies and a Big Bad. Conversely, I’ve been in a single 10+ hour combat with 25+ players on a MUSH.
With grid map combat, you can as a DM engage with the players who want to use interesting solutions based on what they can see and interact with on that map. It’s sometimes hard to do in the mind. It also makes it easier to remind players such things as fireball can hit your flying character if you’re within the sphere of damage as it isn’t a 2D game, it’s a +3D (multiple dimensions/planes) game. Grid maps help identify where the player is in the world and the threats and such are at.
For this, I’d make a grid on the MUSH for normal moving around, but then for a specific battle or encounter, have it on a specific point in the grid map like a battlefield and run it simultaneously on Fantasy Grounds or Roll20 so my cohort of players can focus on strategic decisions which can be helped via visual cues they’re picking up on the grid map. But there's a bit of a learning curve on FG or R20, not to mention making accounts and whatnot.
For anything larger than 6 players and preferably at 40 players, I'd probably want 5-6 players per DM, and an overall DM coordinating and reporting to the overseer on something like an Epics event on a MUSH so things aren't clogged up and folks can see how their group and others are impacting the world. The overseer would emit on the main grid room of the battle, while I'd have players and their DM's TT in that room once combat started.