The Radian installs beautifully in the Safari (ergo the Astro too,) particularly in the center position, but I also had it second row outboard (near the door) and it worked okay. Unfortunately it's far from the recommendation of inexpensive.
The TrueFit goes in with the lap belt only beautifully, and would also work second row outboard, but beware the third row outboard (as Kenah and I have recently discussed, lol.) The biggest hangup is that those belts are THICK, and it can make the lockoffs hard to close.
OP, how old is your DD? Is she in a booster? Just concerned that she was RFing in a TF and now you reference "no other carseats," I want to make sure that she's properly restrained, too.
The other option might be -- if she's in a seat elsewhere but won't be riding in the van -- to turn her seat around RF for the van and buy her a new seat.
Honestly, there wasn't a seat I found that didn't play well with the van when I had it, and it was wide enough for any three-across I could dream up. (RFing, I used a TrueFit, MyRide, Scenera, Radian, and SR22, with and without base. I think an old style Triumph, a Touriva, and an Evenflo bucket, too. Probably more, but I'd transported so many kids in it I don't remember, lol.) I don't miss many things, but in moving to my Pilot's miniature third row -- I miss the width greatly. Not sure when they started putting TAs in vehicles up there, but if your Astro has them they are very small squares on the "feet" of the second row (both outboards) and third row passenger outboard. They're very easy to miss. Or at least that's how they are in the American version.
How much do MyRides or Complete Airs run up there? Might you be able to get one on super-sale, for a decent price?
Option #47 (lol) might be to move your son's current seat into the van, and get him a new main ride... especially if you're interested in RFing quite a bit longer and his current seat will be outgrown (by height, since obviously not weight) sooner rather than later. It might be easier to swallow a higher price tag if you're going to use a new seat on an everyday basis.