I think I like the helper spring option better. However, what is the rating the helper? Is it enough to make a difference?

Have you considered that you may have too much tongue weight? That can cause sway just as much as too little weight. A 19' boat/trailer should only have around 300 pounds of tongue weight max...Grab a friend who, along with you, weighs a total of 300 pounds or slightly more. Both of you get in the back of the van and have the wifey measure how far down the van settles. The boat shouldn't settle it down any farther than YOU do. If so, it may be time to adjust your bow stop---

That said...

Coil-over shocks are okay, but you're still adding stress to the shock mounts, just like air shocks. IMO your safest bet is to add the helper springs. How much they'll help, no clue, BUT they're functional and won't contribute to breaking anything like air or coil-over shocks might. Those pictured are 'half' springs, that mount under the leaf (as shown on the corner of the box)...I've seen 'full' length helpers, but they might not be available to fit an Astro...You may consider finding some that mount ABOVE the leaf, like this:



--I have no experience with either, but from an engineering standpoint, it just appears to me the above-the-leaf style MIGHT be more effective at preventing leaf-sag...

I think the bigger the better!

It's adjustable, depending on how you mount it, to 1,500 lbs. I imagine that will probably do. I checked out a set like FR showed, I think they were good to 1,000 lbs. There's also another set good to 2,000, but the Astro has the axle under the spring, which would require removing the axle to install, no thanks.