When is a WDH neccessary

what truck? I would suspect it will be advantages regardless.

I towed my current TT, 3877# GTWR, 23' long tip to stern, a so called 16' actually 18' rounded front to back, with my 2008 Explorer. A WDH was needed from the squat and steering control.
Not needed with my new F150 but makes for much better towing, no sway, and levels out the truck.
I think, unless you can get a great deal on the WDH with install from your TT dealer, you should try the new TT on a standard hitch first (assuming within hitch weight specs).
I lost one of my spring bars on a trip from improperly attaching it in a hurry. Found out when setting up camp. Towed home with no spring bars, 70 miles one way mostly on interstates. No probs but sidewinds could be felt at times so I kept the speed at 65mph.

A WD hitch is recommended for any trailer >5000 pounds. It may be indicated for a lesser trailer depending on your tow vehicle. I use them primarily to prevent overloading the hitch itself; they are not a substitute for a tow vehicle with proper payload and tires to handle the capacity.

The only reason I use one is to keep the headlights aimed down.
No need to blind oncoming drivers.

Not exactly the route I went but the down sizing was. I had a Motor Home that I was using as my home base. I had bought a 17 foot Casita single axle trailer to travel with. I'm single so room was not a problem. Used the money from the sale of the MH and bought a used 40 foot 5th wheel trailer and had it hauled to my lot, probably never to be moved again by me.

So that left me with a 17 foot Casita/ Tacoma pickup. As far as WDH is concerned, that is a question only you can answer. What I can tell you is the rig rides smoother with than without. I use an Andersen WDH which also has it's own built-in sway control.

A WD is needed when the tongue weight exceeds the manufactures hitch rating for weight carrying ability, when the rear of the tow vehicle sags so much that headlights will not aim properly, when the steering feels light or out of control, when you are exceeding the rear axle or tire loading ratings of the tow vehicle, or when the the trailer feels like it is taking control of the tow vehicle every time you hitch it up, or a combination of all or most of these.

My feelings are that you will probably not need one for the trailer you are considering BUT keep in mind that tongue weights are usually much heavier than described by the trailer brochure or specs by the time you are ready to go camping. Also don't know what you mean by a "considerable smaller truck".
A truck like a Ford Ranger may need one while a F150 probably would not. Many people are quite surprised when they eventually get around to actually weighing the tongue of the trailer.
Barney