Ross did not personally hate Cooper, he "loved" him as much as a narcissist is capable of loving--which is not at all in the sense we non-narcissistic people love people---it's actually tantamount to how we "normal" people "love" objects.
Ross was all about his image, which he would inflate and aggrandize at any opportunity. Cooper was, at one point, a desired trophy and toy for Ross. Initially, Cooper stroked Ross's ego--helping accentuate the good, Christian man, husband, father trifecta he was trying to conjure. But like with any narcissist, the toy (Cooper) lost its splendor and, for Ross, it became a burden. When the burden and hassle that came with Cooper outweighed his benefits, Ross devised a plan that would rid him of the burden without tarnishing his image---in fact, the plan (had it not been figured out) would have enhanced it as he could now add suffering father and hot car advocate to his image.
We have lots of evidence that Ross did not hate Cooper and "loved" him (as an object). Ross was telling the truth in the interrogation room when he said there was "no malice," -- because in his mind he let nature take its course with Cooper, he did not directly, physically harm Cooper in any way, shape, form or fashion. I wouldn't be surprised if in those 30 seconds in the car, Ross was telling Cooper he loved him and he was sorry for what was about to happen to him.
You keep saying there were easier ways out for Ross. Yes, those are easier for US, but they weren't easier for HIM. You are not taking into consideration Ross's personality (disorder). Just walking away from his marriage and tossing Cooper to Leanna would hurt his standing in his church, in his family, and in his community. It also would not rid him of the financial obligations he would have to Cooper--Ross would still have to pay his daycare and likely his health benefits, in addition to child support. Yes, he could have given up his parental rights---but again, that would reflect poorly on his caricature of perfection that Ross's personality craved. I mean, what would HIS BROTHER think of that? Ross clearly (as seen at trial) cares a great deal for what his older brother thinks and feels.
For Ross, the simplest solution was to stage an accident. It wouldn't tarnish his image, it would garner sympathy, it would allow him to seek out fame and notoriety as an advocate for hot car deaths, it would "allow" his marriage to dissolve, and most importantly it removes any and all traces of the burden Cooper had on his life.