You have not reached the top end with tirz yet, so why change now? This takes time.
I've been using roughly 8 to 8.5 mg of tirz each week. I'm only a couple pounds from goal but the scale has stopped moving and even crept up a bit. This morning I reconstituted a vial of each compound and now I'm deciding what to do next. I could use up the tirz over the coming three or four weeks before trying reta, or switch at the next injection. What I've seen suggests avoiding a stack and waiting at least seven days after the last tirz before starting reta around 1 mg. Plenty of people here have experience with both, some stack without trouble while others caution against it. I'm leaning toward switching after the wait.