Welcome to the forum, @cyrilenglebert!
For the full bar rest in the first bar, you can either set as a general option that you don’t want to show full bar rests for inactive players, or ask for it locally with a condensing change.
The option is under Notation Options > Condensing > Inactive players:
If you prefer a condensing change, you can insert it on the first beat of the first bar and set it like this:
Edit: see below!
However, apparently it won’t hide the eighth rest at the end of the next bar (I’m not sure why). For that, you can insert a condensing change (a manual one) on the first 32d note (or before in that bar), to force a single-voice display, like this:
And then, on the first beat of the next bar, a condensing reset:
Yes, I hate this!
I’ve had to deal with this recently and I think another user has brought this up a month or two ago.
So, sorry @cyrilenglebert: you can achieve what you want without condensing changes, just by setting these two options like this:
(The default for the bottom value is 2, which prevents hiding of any rest smaller than a half rest! Set it even to 0 if you want.)
Ahhh! I might have to fiddle with this tonight!
Thank you so much @charles_piano . I love Dorico but sometimes it’s quite complicated to hide little things. I miss my H key on Finale 
