Bonding in general is stressful on the bunnies anyways so I don’t understand how you think the stress bonding is cruel. It just helps them lean together for comfort, making the situation less stressful for them. They are already stressed about the bonding but making it just a tad more out of their comfort zone isn’t cruel. It reaps alot of benefits of you are struggling with fighting.
With that said though, I don’t think you need to stress bond. It’s usually used when two buns won’t stop fighting. I think with time and more bonding sessions they will both grow to trust each other and not act fearful
do go back to the first step in bonding though. They need to rebuild trust for each other before he will stop being fearful. If they fight, stress bonding will have to happen though.
When they were in the bathroom how did they act? What were they each doing? We need detailed descriptions on how each session is to offer advice for your personal situation.