The biggest thing that's going to influence 'flex' (deflection) in this instance is going to be the C/C of the supporting members. The difference in stiffness between 18mm thk. ply X or Y will be fairly minimal.
For screws I use Screwfix's Turbo gold without issue (posi). I don't see the need for ktx or torx for the home user (plus stabbing yourself with a ktx bit seems to do some serious damage in my experience).
Due to a fuck up, our first set on the wall, got fixed with Pozi’s instead of the (much more expensive) Torx I had neatly lined up, ready to go.
We never switched back and have never had an issue.
Used the Torx for wall expansion later. Using Torx for structural work and Pozi for setting, helps (helps, there are some idiots out there) to ensure structural screws don’t get taken out during setting.
Can confirm, using an impact driver, it’s entirely possible to set a 5 or even 6 mm screw spinning in hardwood ply; always assuming the head doesn’t come off first.
On that point: Reisser Cutters from Toolstation, are my choice. Shankless, or you’ll have trouble getting them out again.