Eventually, we want to be able to take the mat along to picnics, restaurant patios, outdoor cafes … let’s work up to that by taking it along on walks and asking our dogs to settle in many different places!

It is totally normal that in the beginning your dog might not be completely relaxed.
If you wean the treats too quickly, they’re likely to get up.
It takes time and repetition to achieve the state where they can truly relax on the mat and are not “actively waiting for the next treat”.

This is absolutely okay and an expected part of the learning process!