Yes, the visual test is not enough. Below a line of tests from left to right:
PETG to PLA 500mm³, 400mm³ and 300mm³ flushing volume.
300 looked fine, but I could pop off the bridge with some effort. From 400 onward I could not, even with considerable effort, break it.
The poop on the bottom left is from the 300mm³ test. On the right the change from PLA to PETG with 80mm³ seems perfect. As DzzD said, I dont mind a bit of PLA in the bottom interface PETG layer.
(eSun orange PETG @250°, eSun whitePLA+ @220°)
BTW, has anyone a recommendation for a better PETG (in bright colours) for this purpose. Speed and adhesion seems not to great with this one.