I could do the design for you if you give me the specs, that would take me some time for which I charge a fee for, I'm pointing you in what to look for, I haven't seen measurements results neither run the sim to make better suggestions. I'm not contradicting myself, drop in replacement of a BJT for a mosfet doesn't usually work, but a circuit using a mosfet tends to be more efficent than one using a bjt. I've also said that bjt efficency is probably good enough, and I think you only need to address the brightness stability, that would make a viable product, all the rest is for later optimization, I don't think you need to learn all about mosfets to get this running.
JS