but once they decide to do teshuva and become frum, they don’t have to do everything at one time.
a ger on the other hand it’s all or nothing
BT can’t do everything at one time, the same like a ger do not learn everything at one time. Not Jew spent a few years to learn and then make a convergin – became a Jew. This is the reason, why it’s all or nothing (Jew or not Jew)
BT IS a Jew, who is getting back to her/his roots. And whatever BT learn, s/he must to keep from the time s/he learn. I may be very wrong, but I think most of BT started to learn from everyday Halachah ( which for women include the law of tznius) and then got farther.