Yeah, 6mg's way too aggressive to start reta with, regardless of prior tirz experience. Better cautious than sorry, particularly given the glucagon component in reta. Personally wouldn't go above 2mg for that first reta dose since splitting the week into two doses works fine (or even dosing daily in tiny amounts). That way you could layer in an extra 1mg toward the back half of the week should 2mg appear to do nothing initially. That said, some GI side effects might take up to five days before showing.
Worst case scenario, better to plan on holding weight steady rather than chasing further loss, at least through this initial transition, focusing on body composition instead helps take attention off the scale.
Biggest culprits behind switch-related problems tend to be either cutting tirz too aggressively (rushing the reta transition) or ramping reta too fast (since it can hit differently). So low and slow tends to be the safer bet, as usual. But dosing multiple times weekly gives you more room to fine-tune along the way.
Always possible to increase reta down the road, whether that's within the same stretch or the following week, adjusting gradually as time goes on. Rarely, some people experience extreme reactions to reta, projectile vomiting or persistent hiccups, even after a careful, gradual approach to