How have people fixed the cable conduit to their chassis. Is cable ties Iva friendly down the tunnel ? Or is it P clips all the way ??

P clips look good but if you need to get at wiring within the split conduit it's a pain. I used self mount cable tie bases which have a barb that pushes into a 5mm hole,the cable tie also has a release mechanism so it can be quickly undone.

Cable ties are certainly cheap and quick, but you have to cut and replace them if you want to make wiring changes in the conduit.

P-Clips might take a little longer to install, but they can typically be accessed/replaced for any wiring changes.

It's mainly I don't want to drill more holes in the chassis than needed and for Iva purposes

assuming you have holes for brake and fuel lines, combine 2 p clips to become '8' clips with bolt/rivet thru the centre.