While I agree with your conclusion as to the "pick of the bunch", I think you have made an error re the best payload when towing at 3.5t. Note the GVM upgrade does not help the Ford/Mazda payload when towing 3.5t. They are ALWAYS limited by the GCM first. Note ranger is the worst of the dual cabs due to it having the highest kerb weight, which leaves buggar all payload when GCM is accounted for (6000kg GCM -(3500kg trailer total) - (2229 kerb weight (XLT, Widtrak is worse) ) = 271kg max payload on/in the vehicle). Tow ball load does not have a lot of effect in this calculation as vehicle GVM is no where near overloaded, so the GVM upgrades do no help here, as they would on a LandCruiser.
Below are revised numbers I calculated from the other 3.5t towing thread, updated based on current models and GCM numbers being now published for 200 series Landcruisers. I have added in a BT50 and SR5 Hilux, and adjusted the comments on the Landcruiser given they now identify a GCM.
All payloads below need to account for all additional accessories (bulbar, lights, wracks, canopies, wheels etc), fuel, passengers, gear etc as may be added into the vehicle. These numbers all assume you are towing a full 3500kg with 350kg on the tow ball. Obviosuly as the tow weight and/or tow ball load reduces, your payloads will improve. and this becomes much less of an issue. Up to say 3.0t, most of these payload compliance issues pretty much go away for any of the vehicles.
79 series single cab 935kg (plus any GVM upgrades if done) (noting this payload excludes tray I believe, so these must be allowed immediately).
79 series dual cab 785kg (plus any GVM upgrades if done) (noting this payload excludes tray I believe, so these must be allowed immediately).
LC200 Land Cruiser 280-410kg depending on model (based on tow ball download of 350kg, plus any GVM upgrades if done will add back to the payload up to 350kg more i.e. Landcruiser with 350kg GVM upgrade will get at best back to 630-760 payload to the vehicle)
Dmax 510kg (vehicle is lightest, thus has best pay load remaining before hitting its GCM).
NP300 Navara 489kg (new Nav is fairly light) - but needs to be then down rate the GVM for tow ball down load so becomes much lower.
New Patrol 395-485kg depending on model (plus any GVM upgrades if done)
Colorado 425kg
BT50 (XT/XTR/GT in auto) 464/395/382kg
Discovery 382kg (based on tow ball download of 350kg and having no listed GCM) plus any GVM upgrade.
Ranger (XLS/XLT/Wildtrak auto) 370/271/229kg
New Toyota Hilux SR5 (manual) 275kg (these have a GCM of 5850 although Toyota does not publish this on their specifications, dealer had to confirm. As the GCM is 150kg lower than most other D/C utes it ultimately give pretty poor overall payload when towing to 3.5t)
This list is generally in order based on best to worst payload... Note if the LC200 Landcruiser gets no GVM upgrade, it drops way down the list to about third last..... This reenforces that clearly there is more to the ability of a the tow vehicle than just the paper calculations. The calculation to me, is more of an insurance exercise for peace of mind.
Interestingly, the lighter dual cab utes (Dmax/Navara NP300) have the best payloads remaining, but are also the ones that appear to be the least capable of genuinely dealing with the 3.5t out back (if you agree with most reviews and opinions out there).