A very interesting dilemma and my 2 cents worth based on what little I know to add to excellent points already made which if some seem to conflict it is probably due to “horses for courses”.
At nearly 13hp/tonne I’d say very well powered so 8 gears would normally be fine as long as you carefully select the best diff ratio depending on whether it is mostly low speed town and country or higher speed distance work.
If it’s a good mix of both then I would fancy a few more cogs with a diff biased towards the bulk of the work. Otherwise the drivability, torque and mpg will take a hit.
As OP says a splitter is great on the top 2 or 3 cogs and its late arrival on the Scania made a real truck out of it.
I prefer the 16 speeds myself because of the H pattern but hardly ever use more than any 10 of them.
I would be doing a few test drives when loaded to get a better handle on this one.
I know you said no to auto but…
some guys do more gear changes in their first hour than others do in a day so this can skew the auto/manual argument.
For days with lots of changes I prefer the Auto box (better for clutch too) and use it manually as much as I fancy but that’s a Volvo auto which is excellent.
Unless the latest ■■■ box that Daf fit has improved I probably wouldn’t go there but on some makes the manual is now an expensive optional extra I believe.