I agree that I'd consider a booster, if she has the maturity. If you want her harnessed, though, that's okay too.
My nanny-charge, M, sounds like she's similarly-sized as your DD -- just slightly more petite. She still has a bit of growth room (maybe .5" or 1" tops) in the RN and is 37ish pounds. This week I've had her FF in a ProSport, and we both like it a lot. She's even debating giving up her "pretty pink seat" (the RN) for it, although I'm not sure she actually gets to make that decision (due to logistics.)
It fits her well -- she's on the second-to-top harness height -- and she can buckle and unbuckle as long as the harness is loose. I reach back, lift the tab, have her lean forward and then she unbuckles herself, rebuckles when we get back in, and tightens it down herself. She has better carseat usage than many adults I know!
You definitely need to be aware of its limitations, but the funky seatbelt install doesn't bother me and I have enough kids in my care -- and seats to choose from -- that I just think of it as a seat with a 52lb harnessed limit (although with the serpentine belt path I would use it tetherless for a heavier child in a pinch, but it's not my first choice.) It has top slots similar to the GN, SK/Maestro, and original Frontier, so it should have good pass-down potential for your other two kids. I'm not sure how it fits petite kids in booster mode.
I got the ProSport because it was the only HWH seat that fit where I needed it, in a tight 3-across, but I have been surprised by how much I like it. It tends to be my go-to seat when I need to FF someone. It puzzles like it's designed to with a RF Coccoro, okay with an RA50, and not at all with a MR. As bizarre as the belt path is, once it's secure (it takes a bit of work) it's REALLY secure and all of the kids I've used in it (4-6 year olds, 35-57lbs) have reported it being comfortable. The sides are supportive and they sleep great in it, never seen any head slump. Harness adjuster is smooth.
HTH