My bet would be rather on the next goddess featured Update.
Gwen never tought MC any spells... that was sabrinas task and she is only a barely talented apprentice aswell.
For Isha MC is but another tool, a new found servant turned into walking power generator with additional function of being an enjoyable fuck. She offered MC her knowledge and balls draining for loyality, obedience, submission and power injection from time to time. But why she should teach MC more? Why she should allow him to be more powerful, give even greater access to knowledge and power if she wasn't sure of his loyality? No, she just gave him but enough to lure him in, a few spells, a place in bed. Then she put him into test, to see if he's truly loyal or not. And MC was on the cross-road, had a choice to make. Isha or Athia. One goddess or the other.
My guess is, if you pick Isha she will teach you herself more and more. Not only in terms of magical prowess, but in bed too.
And if you choose Athia then... you will be both learning magic by trails and errors and with Syfa, if she wasn't banished.