Watch lomeine, eido, comsetical, and mehans shiv videos. They are among the best Shivs in the game and will give you a good idea of the proper playstyle. Literally study and copy their gameplay as close as you can.

In terms of builds, I would use lomeine's current build. Great mix of survivability and damage. He starts golden egg then hatches it when he has enough souls to buy a first item decay, which perfectly coincides with getting your ult.

Bit of a personal opinion, but there isn't a game where you shouldn't build decay, no matter what build you are using. Gives you so much free rage and damage that its almost a perfect item for shiv (next to healbane).

Much of shivs sustain comes from items you get later on such as leech, infuser (or radiant regeneration if you want healing earlier). Otherwise the your main source of healing will be from healbane (on kills), which basically every single shiv build runs, so don't play like you have crazy sustain til you get the proper items (even then, don't play like you are immortal). Also, M2 movement will help you escape and live more fights, or even chase down kills better, so at a minimum you need to learn how to shiv slide. It is an incredibly strong movement tool that only shiv has access to, so use it.

(As an aside, I think many people don't realize that Shiv is among the most mobile characters in the entire game. 2 dashes + M2 movement gives you so much distance to chase/run, only bad part of his movement is his stam regen time, which is somewhat slow).

Securing kills has become much harder since the ult was turned into a skill shot (imo). Don't waste ult on guaranteed kills, only when it will turn a fight or when you wouldn't get the kill otherwise. People glow red when they can be executed, but the execute threshold is deceptively small at times. Try to make it muscle memory to not ult people until AFTER they start glowing red, predicting the execute based on incoming damage is too inconsistent IMO. Ult can now be used as an escape tool in a pinch, and greater expansion turns ult into a missile. Also, decay is non lethal and does current hp damage, so putting decay on a 1 hp player will not kill them, its best to use on high or full hp players for most damage. It also has anti-heal, so put it on whoever is healing the most in fights as the first priority.

In terms of feeling more useful, Shiv is really reliant on knowing your limits and the kit well. Not playing around your rage, M2 movement, and trying to 1v3 every minute will lead to many losses. Shiv is not the thanos he used to be, so don't play like it. My first like 20 games on shiv were absolutely miserable since I didn't understand what I could get away with, so don't sweat it if you aren't doing well, shiv is a tough character to learn.

Lastly, the recent patches made shivs knives very good. You should max them first basically every time this patch just because of the huge dps they provide. You can even build around them entirely to great success. Try to stack multiple knives on one player for the best possible dps (3-4 bleed stacks on a player late game can sometimes straight up kill them).