You want to buy a Graco Nautilus and keep her harnessed until she's at least 4 years old and 40 pounds. Now, if she reaches those limits (must be BOTH!!) and is still not mature enough to sit in the booster properly 100% of the time, keep her harnessed. When I say properly, I mean do not touch the seat belt, do not lean over and reach for a toy, do not slump forward or over when asleep. The child must remain in the upright position 100% of the time, or they're not in the correct position and it could be deadly in an accident. You also want to make sure that when you convert to a high back booster that the seat belt fits properly. The lap portion of the belt should never be on the tummy, and should lay across the upper thighs. The shoulder portion of the belt should lay flat across the chest and over the shoulder, never on the neck. A belt on the belly or the neck could be DEADLY. Trust me, we're going through this with my 5 year old cousin who is in the ICU at a children's hospital suffering from "seat belt syndrome" from her mom not having her restrained properly. It's a sad sad thing, and no one should ever be put through this.
Also, MAKE SURE THE FORWARD FACING CAR SEAT IS TO TETHERED. It's not required, but it's highly recommended as it reduces head excursion by up to 6 inches!! That's huge. Do not ever use a forward facing car seat without it being top tethered. If your vehicle doesn't have tether anchors, contact your local dealership as most cars can have them put in.
That being said, WINTER COATS ARE NOT SAFE TO WEAR IN THE CAR! Ever. Not even for adults. You can't get the harness (or seat belt) tight enough, and it gives parents the false illusion that it is. When you're in an accident, that coat is going to compress, leaving slack in the harness. Best case senario, the child is at increased risk of head and neck injuries because their bodies flies forward further before the harness "stops" them. Worst case scenario, the child is ejected from the harness and car seat all together.
What you need to do is put the child in the harness with regular clothes on. Tighten the harness properly (snug as a hug, no slack around the hips, try pinching the harness vertically at the shoulders, your fingers should slip off and not be able to pinch the harness). Now, take your child out of the car seat WITHOUT LOOSENING THE HARNESS. Put your child's coat on, and put them back in the car seat without loosening the harness. If you can buckle them in, then you're find. If you can't and need to adjust the harness, then you CAN NOT wear that coat in the caar seat. Ever. Period. It's not safe.
Look into getting a nice polar fleece jacket for your child. Fleece is actually super warm. And the great thing about it is that it's not bulky, so it's very safe for teh car seat.
You stated that you can barely strap her in with her coat. That is a HUGE red flag right there. NOT SAFE. DON'T do it! Seriously. We're taking serious injuried or potentially death.
Please visit www.car-seat.org if you have any further questions.
**Adding- The quality between the Frontier & the Nautilus is NOT huge. They're both very safe, and both pass the exact same testing. One is not safer than the other. And, the Nautilus will last longer, for half the price. No child is going to make it to 65 pounds in the Nautilus or the Frontier, unless they're super short. And even at that, there is no need to harness a 65 pound child, unless it's a 4 year old. The Frontier does not convert to a no back booster, but the Nautilus does. And the Nautilus' top harness slot is actually a little taller than the Frontier. There is no need to buy the Frontier, unless you just want a Britax. The Nautilus is very user friendly and easy to install, where the Frontier isn't as easy to install sometimes.
Also, the cheaper harnessing seats that convert to a booster (like the one made by Cosco) have very, very low top harness slots and your child has possibly already outgrown it by height. So the seat would be no good for you. My 4.5 year old has a very short torso, and has already outgrown this seat by height (my mom had one, but it has been replaced with a Nautilus-- we own 4!)