Hello,

In our last session our party ran into a few minotaurs and "stalker of baphomet" enemies while wandering a labyrinth. In the course of the fighting, I noticed that the SoB was wrecking our party with his weapon quite a bit more than Stone Giants or anything else we've ran into has (our Paladin specifically got cornered and messed up pretty quickly). In the course of that fighting, one of these SoB's was forced to drop his weapon (from a Fear spell) and since my PC (Goliath EK fighter - 12 / WM Wizard 5) happened to be close by I ran by and picked it up as we made our attempt to escape the labyrinth. 

At end of session, conversation with DM around this weapon ensues, and they assure me it is something I can use if I want to.

Next session, we find out a bit more about the weapon, and that it can be attuned to. So I say screw it, lets see what this thing that was wrecking us does, and attune to it.

It is a +3 Glaive, with a 15-foot reach, and deals 2d10 slashing damage as well as 2d8 force damage on hit. If you are not size "huge," you make your attack rolls with disadvantage (interestingly enough, I am able to become size "huge" with some planning).

It is also cursed by Baphomet in a few ways I've been made aware of that the rest of the party doesn't necessarily know yet, but they aren't relevant to my upcoming questions about this weapon (One of them is also disadvantage with attacks using any other weapon).

-> I want to make this thing function and read properly in the Character Sheet, if it's possible to do so?

I have tried making a homebrew weapon, but I can't change the damage dice to 2d10 and 2d8, or even just 2d10.

I have made custom actions for each of these damages, but I can't give them the +3 to hit in those entries.

I have tried to make a custom feat with these actions too, but just get more confused the longer I mess around with that and I don't know how to change the reach value that is shown.

So, is there some expert out there that would know how to make this work? Or would I have to live with like the 5 different custom entries I've made and use the parts of each that work correctly, each time?

You'll want to read through Everything you need to know about creating Homebrew Weapons. 

TLDR:

You can't change or add to the damage dice of the base weapon. If you want a weapon that does 2d10 damage, you have to start with the base weapon Rifle, Hunting, add and ignore the appropriate weapon properties (some weapon properties such as range or firearm specific properties cannot be ignored), add the +3 bonus, and change the damage type. Also:

• You will not be able to change it from a firearm to a martial weapon. (You'll need to account for this by giving them proficiency to firearms (done through the homebrew item) or proficiency to a Rifle, Hunting (done through customizing the character sheet))
• You will not be able to change it from a ranged weapon to a melee weapon.
• You will not be able to change the reach to 15' as reach only adds 5' to the character's default melee.
• You will not be able to add the additional 2d8 damage dice. ¹

¹ You can make a custom action called "SoB's weapon of overpoweredness" giving you a place to put the extra 2d8 force damage. This does mean the player will have 2 different buttons to press to roll the complete damage, but this is the only way I see to accomplish this.

Are you playing on a VTT? If so can you make a macro to roll this damage?

Other than that, Mairondil probably has the best solution. 

Giving this some more thought, my above suggestion only works on DEX based weapons. Since the base weapon is ranged, it's going to be based on DEX. I believe you can get around this by giving the weapon the finesse property. You might be able to use the Bonus -> Magic Item Attack With Strength, but that method has it's own drawbacks, as explained in the Homebrew Items & Equipment FAQ, #10 here.

My homebrew weapon experiments concentrated on finesse based weapons so all the DEX weapons worked without issue. Converting ranged base weapons to work for a Strength based weapon isn't something I've completely explored.

Right on. Thanks for the insights! 

Will just be a bit more clunky than I want, then!