When hosting at least with or not many racers, restart are too fast.
If I hit accidentaly my /restart button I have no time to hit my /cv.
Is there an option to set it to 5s minimum?

Thanks

I dont think you can change the countdown. But you can bind a function key with /cv.

i.e. F8 = /cv

So when you mistakenly press the restart and the count down starts, you can quickly hit F8 (or another suitable easily accessible key/button).

I have the /cv binded to a wheel button, but sometimes it is a pain to react in time...

How about eliminating the problem?

Remove the /restart button-bind and simply have it as a keyboard bind (preferably CTRL+ or ATL+) so that you wont accidently hit it and activate the countdown.