I haven’t been able to make this (Glaive Prime) SP viable since I suck at modding, what to do?

You wan't to build for the heavy attack detonation, it'll force slash procs on everything in the radius and clean house. Raw damage, sacrifical mods if you have them, crit chance/damage and heavy wind up. Life strike too if you feel you want the healing.

That’s what the build is for, I’m guessing the sacrificial mods will help. Also, should I mod for slash?
- Slash (and gas) only scale off base damage, crit damage and faction damage. Putting more slash mods on the weapon won't boost it at all, and since the proc is garaunteed you don't need it for the weighting either.

You definitely just need to figure out how to detonate heavy attacks. Glaive Prime is the most damaging melee weapon in the game easily.

Replace elemental mods and reflex coil with gladiator might, corrupt charge and either a bane mod or power throw, also sacrificial mods should be better, make sure you detonate the glaive mid flight with the heavy attack button

Glaive is probably THE strongest melee in the game right now, it one shots most steel Path enemies with ease

Either you go for full slash (glad might+bane for example) without viral or viral, I prefer viral since I'm lazy. Take out reflex coil and put corrupt charge there and it should be more than fine (power throw vs volatile quick return is a preference, I prefer more range). The thing is whether or not you detonate the throw properly - hold e to charge, throw, heavy attack while it's in the air.

As you've mentioned, you're building for slash, so you'll want to drop certain mods that don't fill into the slash formula. The really simple version is: Base Damage Mods Count Faction Damage Mods Count

Running elemental mods will do nothing for you. You've got Viral which is a good thought, as viral will multiply slash tick damage, but you're doing all your damage on a single big hit with very little status chance. You can actually drop your elemental mods and pick up a faction mod (primed if you have it, it makes a big difference) and run both critical damage mods. Or you can keep it a little versatile and add in a toxin mod to deal with corpus without having to switch your loadout.

Smite (aka Bane/Faction) mods are important because the actual formula for slash ticks is: 0.35 x "Modded Base Damage" x (1+ Faction bonus) x any additional multipliers. But "Modded Base Damage" also gets multiplied your faction bonus, so base damage mods like pressure point only apply once, but things like smite grineer apply twice, and are multiplicative to base damage.

Here's a build I use when I feel like having my glaive prime out, I usually just bring it on builds where I mostly have it handled with other weapons or abilities but keep it as a backup.

You can also combine this with an external viral source to magnify your bleed damage. Epitaph is the gold standard for glaives since it helps you blanket an area, but a lot of stuff works. These days Nourish is a fantastic option for passive viral priming because it adds viral stacks to everything around you when you take damage.

This build here but personally, swap in amalgam organ shatter and primed fury over the smite. Faction mods are inconvenient and don't do bonus damage to thrax or acolytes, but you can throw almost twice as often when adding both these mods. There's really no justification not to use amalgam organ shatter tbh.

I personally don't notice a difference between primed fury and quickening, or adding another attack speed mod on top of p fury. However, the difference just having one attack speed mod just feels amazing and gets you in a natarak like rhythm.
- Yeah, this is an old build but Tadiken is right about Amalgam Organ Shatter. Miniscule critical damage loss for a lot of comfort. 3.8x multiplier down to just 3.7. I'm pretty sure attack speed mods like Quickening/Fury don't apply to glaive throws, but they do obviously matter if you want to use it as a hybrid light/thrown weapon.

Don't use elemental mods on glaive prime. The only thing you care about boosting is Glaive P's forced slash proc. Things like Initial Combo, Crit Chance and Damage, Faction Damage, and Melee Damage are the only thing that interest you.

Here's (in my opinion) BIS no-riven glaive 2 forma build. Any mod you dont have can be replaced with its regular version, but if you can, get Amalgam Organ Shatter ASAP due to extra wind up speed. If you wanna make it 1 forma you can just use primed pressure point, the difference is minimal.

Build

Another thing worth getting is some kind of Viral-spreading secondary weapon. Or the Panzer Vulp pet.

You throw the glaive by holding melee key, then detonate it midair with Heavy Attack button. And everything dies.

Generally speaking, if you’re building for viral you want viral PROCS, so you need status. Instead of using the 90% element mods, use the 60/60 mods.

You want initial combo count so you don't have to worry about building combo. Easiest way to get it is with the mod Corrupted Charge. Use it instead of Reflex Coil. It'll give you a 2x multiplier on all your heavy attacks without needing to build combo. Try to get Amalgam Organ Shatter. You want viral procs, so replace your 90% elementals with 60/60s for more status chance.