There's certainly no *need* to buy a dedicated booster if you have a combination seat (so long as the booster portion fits properly on your child in your vehicle of course). But the dedicated boosters are generally lighter and smaller than the combo seats, because the booster portion does not have to actually restrain the child. And may be easier to use.... we found the same thing, my 6 yr old could not buckle the Nautilus either because of the big arm rests. And in the truck, it was a huge pain, because I couldn't reach to buckle either (too high up) w/out climbing half-way into the back of the seat...and then I'm half-suffocating the poor kid trying to stand in front of / lean over him and buckle him. In the car that wasn't such a big deal, I could easily reach over and buckle him up myself....but ended up needing to get a regular booster to be able to fit 3-across when #3 came.
For smaller kids, Maestro is good and cheap. The booster portion doesn't last much longer than the harness from what I've seen, so you may need another booster after that.
Frontier (85 or 90) are good seats, but expensive, and the high harness limits are probably overkill for such a small child. (although there's no harm in keeping her harnessed as long as you both are comfortable with it). Probably easier to use as a booster than the Nautilus since the arm rests aren't so big - it'd depend where your seatbelt fits and how long the stalks (female end) are though, since it's a very wide seat. Some seatbelts come out from behind the Frontier and that makes buckling (in booster mode) more difficult.
Instead of the Frontier, you could consider a Britax Pioneer 70 - ($170 on amazon now) - very similiar to the frontier but a bit smaller, lower weight limit, with a better price tag. (it's about 2" shorter than the Frontier, both for harness mode and booster mode, and 20 lower weight limit). Still very respectable top slots at 18.5". Your DD would be well into booster age by the time she outgrew that.
There are the Kid's Embrace combination seats, with characters on them. They don't have as high top slots for harnessing as other combination seats, so harnessed mode will be outgrown sooner, but would work fine for a kiddo on the smaller side like your DD. Probably similiar issues to the Nautilus in booster mode - substantial arm rests that get in the way. (Note - I have not seen one of these seats in person, so could be mistaken). $100 - $150 on amazon, depending on style.
There's also Evenflo SecureKid 300 ($95) or 400 ($115). Again, less height than the Frontier or Nautilus, but better price.