There’ll be a void behind the plywood so you don’t need to worry about the screws being so short as to no go through. You’re not going to get a wood screw thats useful at less than 12mm long really, and if the ply is thin you want the screw threads gripping the ply all the way through, rather than having a screw thats mostly the pointy bit.
As an easily available screw the ‘Gold Screws’ sold in B&Q and Screwfix have a nice deep thread to prevent them pulling out, don’t bother with the ‘Gold Screw Plus’ or ‘Turbo Gold’ as the latter in particular has less bite.
Personally though I’d be using the self drilling screws in Screwfix that are used for fixing flooring to steel – then screw through the ply where theres steel behind to get a proper fix