I have sympathy for that goal, but I think you can get there by refluffing the spear to have a more Glaive-like head.
If there weren’t variable striking ranges for melee Weapons in the game, sure. Just like if all ranged weapons had the same range, I could model any thrown weapon with a handaxe. But those mechanics are in the game, which means that a spear will never
play like a glaive. For me, there is literally no value in having those mechanics if they don’t serve to mechanically differentiate between thematic concepts. The game would benefit from eliminating them, if not for the value of mechanically distinct thematic concepts.
Except you seem to really want reach, too, which strikes me as more about optimization than concept. (No offense intended; I often do the same thing.)
none taken, it just isn’t true, at least in the sense of wanting a more powerful character. I do often try to “optimize” mechanical distinction, which is what I’m doing here.
This is part of what I meant by game terms versus historical accuracy. The weapon you are envisioning should, logically, have a reach advantage over, say, swords. But so should a staff or a spear. Unfortunately (or fortunately?) that's just not how the game uses the term 'reach', when applied to a 5' grid.
but it does distinguish between a spear that is used one or two handed, and a longer weapon that needs two hands. Bc it does so, I can’t escape the knowledge that what my character is doing in the actual game does not match what the concept suggests, even though the mechanics exist for it to do so.
You could take the Mobile feat; that could give you the feel of striking from beyond reach.
how so?
EDIT: Another option, if you're going to homebrew anyway, is make a subclass that specifically uses polearms. That way you can bake in balance and not have to create a weapon that is strictly better than anything on the list. The 3rd level ability could simply be, "Polearms are now monk weapons for you."
EDIT 2: I might add, "...and they are not considered heavy when you wield them." Just to prevent yet another cookie-cutter GWM build.
A d6 finesse polearm that isn’t heavy, and is a monk weapon, isn’t better than any martial weapon. My proposed spiked chain is, but it’s also only available with special training, not as a starting weapon, in my current game.
If the dueling glaive or naginata or whatever deals 1d8, it’s too close in damage to heavy reach weapons, but even then it can’t be used with GWM’s power attack. Being able to have reach and a d6 die with dex isnt overpowered.
EDIT: Might still make a Sohei subclass that combines polearms and divine protective magic.
Or a Matukai-style subclass (Star Wars Force tradition) that can wield their attuned polearm one-handed, and gets weird tricks like hardening their body, heating their hands to deal fire damage and set things aflame with unarmed strikes, and maybe the ability to grant a couple Ki to an ally as part of a long rest, giving them a small pool they can use to do the level 2 monk ki abilities with.
Ita late, though, and I’m not gonna try to figure out the balance point there, right now. Luckily, monk subs have a decent amount of kick, so you can get pretty creative.