Simple: let him continue to play/develop at Iowa. That he's changes his launch angle, FB/GB, and HR frequency reflects that he's still in developmental phase. Iowa remains good for trying to optimize swing decisions and contact capacity. Leave him alone and let him develop.
After that, it's kinda out of Cubs control?
Will he improve enough to develop trade value? If so, great, trade him. But Kevin needs to earn a trade, and other teams to scout his value. Cubs can't make him better or make teams want him, they just wait and hope.
Cubs also don't control Ramirez, Rojas, Triantos, Shaw, Kepley, Conrad, or what their trade values will be. Decisions with Kevin are impacted by progress and Cub-analysis with those guys.
HR's have value; Kevin has been good platoon. *IF* he earns neither a trade nor a starting opportunity, having a bench/platoon future might work, who knows? Again we wait and see over time. No need to write him in as a starter or write him off if you don't.
Guys with bad contact and long swings, that rarely goes away. Fair chance he'll never be more than a 4A, and the Cubs will never get much value. But, only time will tell, so for now we wait and hope.
We do have other internal options for starting, plus we have considerable spending capacity. Between Seiya, Happ, and FA, there is no obligation to go with a prospect, *if* they don't have a prospect they like a lot. And there are enough prospects that they can do some mix-and-match. Likewise it's also possible that *IF* they go internal, that next year's guy doesn't need to be the long-term. For example, maybe Conrad gets healthy and ends up being long-term guy, but isn't ready to even share a spot until next July or the following season. Who knows?