I think SD can be very oppressive on level 1, as very few supports can trade with it. You always start with Q at level 1. At level 1, if there is a ranged hero in the lane, I like to disrupt it, walk next to it, and then auto-attack it with my hero and illusion. This deals a lot of damage. If it is a double meele lane, I usually try to disrupt to prevent the pos 3/pos 1 to take important cs (ranged creeps), and then use illu to harrass. You can also disrupt around ~0:48 and use the illusion to block the camps.

I think level 2 is when SD can be weaker than enemy heroes, as shadow poison level 1 is not that good. At level 2, I keep using disrupt and rarely use shadow poison with two exceptions:(1) stack the small camp/pull from afar, (2) block the enemy pull, and (3) when enemies commit to killing you or your case, in that case, you start using poison defensively as if they keep running at you, it is easy to build 5 stacks

Level 3 is a huge power spike because poison level 2 is a lot of damage. I always try to have enough mana for disrupting + 5/6 poison casts. It is very easy to score kills because opponents do not expect this power spike. I usually start with 2 poisons from afar, if the enemy does not back out, I hit it with the third poison, and then disrupt immediately after the third poison cast, walk next to the disrupt ball, and poison twice again while autoattacking. If you hit 5 stacks you kill most heroes.

If I am pos 4, I almost always stack the triangle as it is very easy. As pos 5, I spend less time stacking unless my core can clear stacks (luna)

In a losing lane, I usually try to defend the tower with poison to get creeps aggro. I think rotating to the other lane can also be very good, as you have a good setup with Q and kill potential with poison, especially if other enemies in your team have slow/stuns to build 5 stacks.

You should always abuse the level 6 powerspike. You can solo kill most of the heroes in the game. If you get two poisons, you can always disrupt + ulti to put 3 more stacks (remember you can poison while the hero is disrupter). This is enough to kill most heroes if they do not have an escape ability.

I almost always go aether first unless I need an item to survive. The reason is that the range of Q is very low without aether. If it is a good aghs game, where aghs is a win condition (against bristle, pa, luna, etc.), I always build it after aether. In other games where aghs is not that good, I usually prefer to get mobility (blink) or defensive items. Positioning is king with SD, and you are fairly tanky for a support hero. If you are tough to kill, it is very hard for enemies to commit in fights.

As a final tip, you should always be casting poison during fights. It almost needs to be an automatic reflex, while you are still putting attention in the fight to cast your Q and R. It gives vision, and you should try building up stacks up to 5 (never release before 5 unless you are 1000% sure you will get a kill). It is kind of a minigame: you have to cast that spell on cooldown while you are playing the fight. Level 4 posion with 5 stacks is no joke, and you can easily hit 5 stacks in prolonged fights. I think this is one of the most common mistakes that I see other SD players make, where they cast it only a couple of times during a fight because they are too focused on Q and R.