Welcome to RevWarTalk, thanks for visting and sharing your post.
Genealogy is an exciting venture. I have limited knowledge of Hessian regiments, though believe that the Regiment von Rhetz came from the Principality of Brunswick-Wolfenbüttel (some more info about it on wikipedia: https://en.wikipedia.org/wiki/Principality_of_Brunswick-Wolfenb%C3%BCttel.
I don’t have a regimental muster roll, though there is a site that has good info on Regiment von Riedesel: http://vonriedesel.weebly.com/history.html . They seem to be one of four Hessian regiments sent to Canada (Regiment von Rhetz another of the 4). This would tie in with your mention of Johann being in Candada after the war. Maybe the folks at that site have a deeper Hessian regiment knowledge, and can point you to some good resources.
If you find anything good please share, it would be great to learn more about Johann.