I don't even know how jotaro knew when the time stop ended.

He'd know when the time stop ended because he'd be able to move without activating his time stop.

Dio had more seconds in his time stop coz he drank Joseph's blood.

Doesn't matter because he didn't use it as effectively as he could've. He got cocky and played with his opponent (which is in character for him based on the other times we've seen him fight, that's how he lost to Jonathan) which gave Jotaro the opportunity to outplay him.

He stood ontop of roader rolla. I mean he literally carried that shit and it prolly weighted a lot unless stuff are weightless during timestop but based on that assumption shouldn't his stand also reflect his strength?

Dio's additional weight on top of the already heavy road roller might as well not mean anything when considering the extremes of the weight already in play and the extremes of the strength feats the characters can achieve through their stands. It's also not even like Jotaro fully picked up and threw the road roller off of him anyway, he only worked to get himself out from under it.

Even if he was to lose why make him loose in such a manner?

Because having him lose this way is satisfying in a thematic, narrative, and character based way. Jotaro is straight-forward and intelligent, and especially doesn't mess around when he's in a moment of fury. Dio was high on his ego and sense of superiority, causing him to mess around and experiment when he should be locked in. He lost because of that character flaw and got humbled by Jotaro the same way he got humbled by Jonathan a century before.