From the size of the chomp on that rose bud, that is way too much for thrips and the slugs I've known haven't gone after buds.

Look for a caterpillar that is probably hiding along a stem. It may be the same green aw the rose stem.

If the bloom had opened, with that kind of damage I've often found the caterpillar inside the petals, but I think your petals are too closely spaced right now for a well fed caterpillar to hide there.

The one thing that's missing is the frass left from caterpillar feeding.

Organic solution: two fingers and squeeze.