Is there some trick to get better alignment of feathers towards the surface ?
Red are my guides (and those are already quite hard to align it seems ?)

In theory my guides are perfectly aligned since I only planted them and bent them downwards

Make sure you click "Set Feather Settings" on guide groom before you draw guides, that should make the barborient attribute on guides align to the surface normal.

When you already have guides, you can use the orient tools to do this.

Also check the "Orient" submenu in the groom radial. Or just pick the "Orient" tool from the "Tool" dropdown, then check the "Direction Mode"s under "Brush Options".

With the "Rotate" mode you can just brush across guides to rotate them, but you might get there faster with "Align to Skin" or "Face Camera".