Then FF to the limits of the seat (not always feasible if you have a 40lb 8 year old etc). FF is outgrown when the weight limit is reached OR the tops of the ears are even with the top of the seat shell.
Also, it's outgrown FF when the shoulder goes over the top harness slot (this usually happens before ears approach the top of the shell).
Basically, best practice is rear facing to the limits of the convertible seat (birth-3 or 4 usually), forward facing until the child is emotionally and physically mature enough for a booster (5-7 years old, usually), boostering until the 5 step test in passed (10-12 years old, usually). High back boosters are preferred for younger booster riders because they give more support to kids when they sleep. Many people put a 40 lb minimum for riding in a booster- in some countries and some states this is the law, but most people here are comfortable with a 35 lb 6 year old in a booster.
At this point, for you, your seats may last until she is booster ready, probably around 5 or 6. In that case, you can expect to buy a highback booster at that point. You might buy one that converts to a backless booster, in this case it
may be the last seat you ever buy.
If she outgrows your current seats before she's booster ready, you will buy a combination seat (harness to booster). Some combination seats make good boosters, some make poor boosters, so you may or may not have to buy a booster after she outgrows the harness. Also, if the booster does not have a backless mode, you may have to buy a backless booster when she outgrows highback mode.
Oh, and I wouldn't get a radian. There's no need, you have two convertibles, both of which she should fit both rear facing and forward facing.