I agree, the rear springs are too soft for towing. My snowmobile trailer has a tongue weight of about 200lbs, plus all your gear in the rear, makes it sag more than ideal. Not that it causes any significant problems other than your headlights are aimed too high if towing at night. I have the Sport which is supposed to have a stiffer suspension but I bet this is just a shock configuration vs. stiffer springs.
You can get air bags installed aftermarket like your old van had but I have no idea with this costs. Weight distribution hitches are an option but should this really be necessary for only 200lbs. Besides, there is debate whether or not one can be used for a unibody vehicle.
I’m considering a boat which will weigh 3,500lbs dry which I’m guessing will have a 350lb tongue weight and I’m concerned about sag.
I was going to recommend a weight distribution hitch. I have one to tow my approximately 4200lb loaded hybrid that likely has a tongue weight of around 500lbs and it is as level as it is with nothing hitched to it. In fact, the rear end sagged lower when I towed my 1800-2000lb hardtop (likely around 200lb tongue weight) without a WDH.
Ah yes, the old weight distribution hitch (WDH) debate. There are plenty of us on here that are using them. Most of us have come to the conclusion that it is simply a Toyota 'cover your ass' clause. I've seen lots of minivans use WDH for years with no issues. Towing an (up to) 5000lb trailer without a WDH with sway control, in my opinion, is like skydiving without a parachute. The tail will wag the dog, so to speak, whenever you drive past another high sided vehicle. The fact that Toyota have been marketing the HL as having a 5000lb towing capacity (properly equipped), but do not recommend using a WDH is pretty stupid in itself. One recommendation that I have read that makes sense is if your trailer weighs more than 50% of your towing vehicle (in my HL Limited's case, 4321lbs x 0.5 = 2160lbs.....maybe a little higher when you factor in cargo and people, but even based on the GVWR, based on this assumption, you shouldn't tow anything above 3000lbs without a WDH.
Anyways, that's my $0.02. If after a few years, my use of a WDH twists the frame of my HL or rips it apart, I will be on here to recommend against using a WDH with the HL, but at this point, I am not concerned.