Yeah sejuani is not a hard lane bully, I definitely would prefer the ganks early from the jungle as sejuani can snowball hard with some help. I will put that into the guide, I would suggest building two tank items first as nasus's q deals physical damage so if you can mitigate his q damage he cant kill you so you might be able to 1v1 him mid game with a sunfire cape; dead mans plate; haunting guise. in this match up its more about your teamfight capabilities, you will have more impact then he will.

Nasus is not really a 3 lvl threat, much more for Sejuani. Maybe you were against a very bad Nasus or something.
You can't really bully him out from lane, he'll be freely stacking Q under his tower (because you'll definitely push with your bullying). And he can stuck under tower all day, denying some your creeps. After 1st-2nd back he'll be able to stack at least 5 of 6 creeps under tower.
Your main problem is that you can't poke him and make his hp low to dive him with your jungler (it will be free doublekill for him). So, at 10:00 you'll have Nasus with at least 150 stacks (if he's good enough) and you will definetely not outtrade him in any way. At 20 minutes he'll be able kill solo both you and your jungler.
As you wrote, you can just roam and gank other lanes, but it's risky. If you'll not succeed with ganks, you'll waste a time and fall more and more behind every second while Nasus easily becomes monster.

The good strategy in NOT to bully him early. His Q will have a pretty long cd so it's ok to let him stuck Q a little bit. You main aim is TO NOT push in any way. Sacrifice some farm, but not let your creeps push. Don't attack him at all. Good Nasus will provocate you to hit him, because after that his minions will start to attack you -> your creeps will attack them -> your creeps will win and push the lane. #minions_psychology, lol.
Take Q and E for CC. Then, if you have a jungler like Lee Sing or Vi - Nasus should be ganged at lvl 2-3. And killed (he's squishy early). [btw, if he's smart, he'll understand what you're trying to do. You need a really good and strong jungler]
Then he should be killed with jungler again immediately. After that just destroy his tower and start roaming.
He's now behind and you're fed and ganking. If he'll return top and farm again - just kill him. Do it every time he's trying to farm. I will be easy without a turret and your level and gold advantage.
If he doesn't farm, he'll not deal a good damage and will become a support-like Nasus with his debuffs from W and E (or he can just leave the game, lol). That Nasus is not OP. That will be the moment you succeeded.

Btw, you don't need such strategy against not good Nasus, but it will be great anyway.
And, also, don't do it without a good jungler (premade mb). Without him YOUR strategy is better than mine.

Good luck ^^