With your keyboard if I want to make « ê » I press shift+6 in order to have the « ^ » and the key « e » right ?
No, Shift-6 just gives ^ as an ascii character. You can press Compose, then Shift-6 and "e" to get "ê", but that's a bit cumbersome. That's why if you often need circumflexes, it's better to have a dead circumflex somewhere (e.g. Meta+V or Meta+ B) , or even map precomposed circumflex letters, e.g. you could map éêè to Meta+WER for optimal efficiency.
Here's my proposal, with the default mapping indicated in a darker shade of the auxiliary color:
http://www.keyboard-layout-editor.com/#/layouts/86df760fd41803365c8390943101d4e1
It has dead circumflex at Meta+B (for âêîôû) and dead caron at Meta+V (for čšžřňľěǧȟŤĎ). The caron is quite important for the Balto-Slavic languages, so it makes sense to have it too.