For the Pallas seat, you can keep using the impact shield to 18kg - but I agree - you'll need a 25kg harnessed seat at some point. You've had some good recommendations on this thread, however I would mention that the harness height on Maxi Cosi Beryl is very short, it's also a very expensive seat, so it's not the best choice for someone wanting to harness to 25kg as it probably won't get you all the way there and your DD is very big for her age, so you defo want a seat that will get you all the way there.
Personally for forward facing I'd look at Joie Bold, BabyAuto Dupla or Cozy n Safe Hudson if you're really on a budget/need something lightweight. But they all need top tether. There is supposedly a version of the BabyAuto listed in a couple of sites on the net that is belt fitted only, but I don't know where it's come from, possibly the Asian market, and I don't know whether I'd trust it.
If you don't have top tether in your car, my recommendation would be a rear facing seat such as Britax Two Way Elite or Max Way. There just aren't any good, affordable options for forward facing over 18kg without a top tether.
It's really really unlikely she'd sit properly in a seatbelt age 2, and there is a school of thought that younger children's pelvises aren't developed enough to prevent them from sliding under the seatbelt (submarining) in a crash, which causes terrible injuries. It would be legal (as she's over 15kg) but probably not safe. Crash test dummies don't get uncomfortable and wriggle and slouch, toddlers do.
If you can get her to 3 I'd be happier, although really 3 is still very young for a seatbelt and most kids I've fitted at age 3 are going crazy taking it off, wriggling around, not sitting properly. I wouldn't argue over a couple of months if she was nearly 4, but generally 3 is still young for a booster. If you did go to seatbelt at age 3, I'd look at the Britax Kidfix seats which have the crotch guard to help avoid submarining, which is a bit of a pain given you've spent the money on the Cybex (which is actually a really good booster seat, just not necessarily for a child who's too young!)