Wow thanks a lot guys for all those quick answers ! Too many answers to quote each of you, i'm sorry.
Is there a navigation app someone can recommand that would work this way : plan the route on the PC, load it in my smartphone, and I would be putting my phone in a rear pocket and listen for insctructions ? I use google maps on the fly, but clearly the routes it recommands are not always suitable for my roadbike.
To answer to people suggesting using a paper sheet, either written by hand or printed : that's what i'm currently using, I think there is a nice vibe following road signs, not using technology and asking people around for directions, but too many times I've been missing a turn or something and extra kilometers are not always welcome! A couple of times I also had to plan the route last minute.
I'm sorry I did not mention my budget, it's around the 100€ mark, which is not much for this kind of product, the lezyne macro GPS is a good option, i'm also looking at the Bryton 420E which seems a bit better.