You're at goal weight and got there with a silly low amount of Reta, good for you.
I've bridged from Tirz to Reta, but I expect to bridge back to Tirz at some point.
You're taking tirz presumptively to control appetite/food noise. Remember it takes ~24 hours for the injection to peak (Tmax) and start with your .5mg idea, sit with it and see how you feel. Take another dose when it seems like you're getting the twangs of food noise or appetite beyond what you're wanting. Keep notes of when you take what you take and let your appetite lead the way vs the calendar or fixed schedule. After you see what you need, develop a schedule around that.
For me I notice food noise coming back before hunger issues (hunger was never really my issue, it was the "I could eat" noise). On the bridge, when I noticed myself thinking about food, I'd take a bump of Tirz to control the noise while Reta moved to a therapeutic range... it took two pins. I'd also take it before a known "stressor" to give me additional capacity, although it proved to not be an issue.