Advice for a new Sigma player?

Hows it going y’all. I’ve played overwatch alot on and off since 2016 but never could understand the dynamic or swing of team shooters until marvel rivals came out. The 3rd person perspective and everyone starting from the ground us has made it alot easier for me to digest the game and i decided to give it another shot with the knowledge i have now.

I mostly play magneto in rivals and sigma seems to translate well with a similar play style and cooldowns. I understand the gameplay loop but i feel lost on a few things still and would love any other advice from players who have been at this longer than i have.

My main issues have been my ult timing, im unsure when i should be using it and who i should prioritize. I understand it takes up to half the max hp of a character when the slam is finalized and i understand movement based characters can escape it but i feel i’m dying or getting stunned out of it too often.

I also struggle against doomfists, I feel he always charges at me faster than i can throw my rock to stun them.

Hope everyone is having a good day and thanks for reading!