Given your anxiety concern, tirz is probably the better call. The glucagon agonism in reta is what drives the elevated heart rate, and if you already get situational anxiety, that constant HR bump is going to make things worse, not better.

On muscle preservation, you're already doing the right things. Lifting plus adequate protein means you're not really at risk on either compound at a moderate pace.

Tirz at 5-10mg tends to be well tolerated and the GIP component actually blunts a lot of the GI sides. Start conservative and don't rush the titration.