Can anyone tell me what is the native disc size for the front and rear? Both 160, 160f/140r, or both 140? Or else, would I need an adapter to mount 160 both front and rear?

I have a shimano 105 groupset .
Front is compatible with 160 but you'll need and adapter for the rear if you want to use 160. Otherwise it seems to be 140 without an adapter.

Not quite sure what you mean by external and internal button, but here's a photo of the new "v3" eR9 shifter from a set that I just got. Apologies for the cling film.

Is there a 2025 v3 version also on the er9 ?

Urgh, mechanical will always be challenging with these frames that are meant to be used with electronic. I would still try to have 2 lines on each side of the stearer - similar to my picture but just 2 lines on each side instead of one each. That way you should have a bit more space for each line. Have not installed mechanical on this, so no hands-on experience, just throwing out some ideas. I think Serge hast some experience with it.

Thank you for your help ! I will give it a try

How do you manage cable routing near the steering tube ? I put all cables on the right side and it kinked my front brake hose . Putting 2 cables each side seems impossible.

Hello guys !
My shimano 105 mechanical front derailleur comes with a cable stopper . Should I run the hose all the way to derailleur and remove the frame cable stopper ?

Thank you !