To be fair, I've found that Sheen alone solves all my mana problems in lane, you typically go OOM only if you spam your E which you shouldn't be doing in the first place. The triforce route thus, has no such problem for me. What really is an issue though, is that, as you say, TF leaves Nasus squishy, which is solved by beefing up first before attempting to finish it (thus the old standard of rushing FH and Visage, leaving sheen as it is for the time being).
As it is now, the 10% from runes seem to be the only way to go with regards to this, since the options now are effectively:
A)
B)

As a sheen item and Visage are considered core on him, this effectively rephrases the question as: How can Nasus effectively get 10% CDR?
A few options have come up, including what I've gleaned from here are:
.
Thanks to RockandRoll for this suggestion, I never even realized Warmogs got CDR tacked onto it now, though I seldom bought it in the first place. That being said, I believe that stacking too much HP on a drain tank isn't too great, since each burst of heal contributes a bigger % to your total hp if your hp is low. I'd be looking at the damage reducing side of things.
This is what I believe to be the best option for now, a tank item that gives resists and CDR. Double locket on the team never hurt either. The problem is, it is highly situational since you wouldn't want to be stacking MR against the common AD heavy comps nowadays. This is my only gripe with this item.
This, while being heavily offensive, I think the active can be worth getting as I've been cc-locked to death in some games. Before anyone says anything about being not tanky enough, this happens even with a full tank IBG pathway. You just get cc-locked for a good 5-6 seconds while overpowered Dominik's and BoRK chips your entire HP bar away. That's the reason I've actually experimented with a
setup occasionally.
I was interested in this, as I saw another Nasus player build it. Granted, he was exceedingly fed so it might not have mattered whatever he built at that point, but reading the tooltip, it actually makes sense on Nasus. Damage reduction is very powerful on a drain tank like Nasus, having an amount of it converted into burn would allow him to just heal back up on it which is what he does as a drain tank. Haven't tried it though.
This was a relic of the past, as another full damage item allowed on Nasus. I still use it though, and it works out. The only part that is challenging, is planning the build path, since you need to know when you're tanky enough to buy some damage followed by when to stop that progression to quickly rush another chainmail if demanded. All the stats it gives are great on Nasus, especially the active.
I'll keep trying out different combinations, for now, it seems like the rune 10% is indeed the way to go. I just dislike having so little MR at the start of the game though, it occasionally makes things rather difficult.