Time to switch TODAY. OTW an accident with straps below her shoulders can (and usually do) snap children's spines from the directionality of the pressure caused by impact + low straps.
It's called "common sense exemptions". We actually had to get a note from the fire department child safety seat inspectors (that I kept pinned to kiddo's seats, in case of being pulled over) when HE was little because we had to turn him FF at right around 6mo, and had to switch to a booster right around age 2. ((By age 2 he was well over 30lbs, and over 3' tall)). I switched too late as well (inspector was horrified that his shoulders were 2" above the highest holes, and took them off the seat right then and there)... so DON'T feel bad. Rules are made for the *general* populace, and we have a very diverse populace (My grandmother, for example, doesn't "qualify" to sit in a car without a carseat according to height/weight laws).
Most states do NOT have an infant/ child/ booster seat by age law. Instead they have wording that says "appropriate carseat" until x height/ y weight law for precisely the reason that due to genetics kids are different sizes. Ditto, children in the backseat usually has a caveat "where practical" (which exempts cars / trucks without back seats as well as when you're transporting more kids that seats in the back as well as when you're transporting something that would make it dangerous for your child to ALSO be back there -tools, boxes, animals, anything heavy/sharp/etc that could squish them if it fell on them OR that takes up the entire back seat-, or also if there's some kind of mechanical failure (like the belt clip is broken, or the seat has been damaged in some way).
IDEALLY everyone should be in a 5 point restraint, regardless of age. And for about $500 per seat, you can retrofit your car with "racing straps". But the fact of the matter is that we don't live in an ideal world. If your daughter slid down during an accident in a booster (which are usually angled to prevent that a LOT more than a regular seat) the most common injury are broken legs. IF however, in an accident her straps are too low, the most common injury is a severed spine -paraplegic, quadrapalegic, or fatally. Do what's right for YOUR daughter's body (and get a note from a seat inspector if you have to), and avoid everyone's "horror" at not doing what they do for their SMALLER children.