A lot of people find that the skin sensitivity problems eventually fade, so one option is to simply drop back to 3.5mg reta for a few weeks and check whether that brings relief.
I haven't taken this path myself yet, though it's been on my mind quite a bit. Since you seem to react more strongly to reta than most, I'd move into tirz cautiously, in case you turn out to be sensitive to that too. Tirz delivers more powerful appetite suppression, so don't treat the two as a 1:1 ratio. My suggestion would be to reduce reta by .5mg while adding .2mg tirz during week one. Pay attention to how you feal, then likely keep going each week at roughly that same rate. Tweak things if necessary. Appetite should, I'd guess, remain fairly steady. Continue taking these steps at whatever speed feels right for you, until the results satisfy you.
Let us know how it goes.