We started with Gaia when we got her (7 weeks).
Our breeder warned us for her to escape, but we thought: well, we try to train her in a safe environment and see how for we can get.
No shooting is certainly missed (dutch expression)
We went to very quiet places with no traffic at all.
When we were sure that there were no (other) dogs, humans etc. we let her off leach.
Then, at certain moments, we played hide en seek.
When Gaia had no attention for us, we quikly stand behind a big tree.
We saw that she became nervous and then we called her and rewarded her.
Later on, we learned her that the dogwhistle (we bought) means that there is something nice to get.
First we trained this at home.
And that something must be special!!
I understand that you are very carefull, and we would be to if our Gaia takes the "kuierlatten'" (dutch expression) but so far she listens, except when she plays with other dogs.
If the other dog leaves, Gaia goes with her / him and we have to walk back to catch her.
In Monster, a little town, just 5 kilometers from Naaldwijk, there is a place 1,5kilometers long, totally fenced ,where the entrance locks behind you.
Something like that is a perfect training place.
I hope you find something for Mirtillo!