I am curious why you think his daughter wouldnt have control issues since it is the flame that seems to be the problem which she also has not sure how her mother would help that. like i said he seems fine unless he needs to pull from his core the fact he learned the summon in a couple of days shows this imo.
From what i took from the story the mc is able to use his flame always with no to a little energy cost. he can combine them (firestorm) for a decent amount of energy but to be able to use just the arcane he needs to seperate it from his flame compress it in his hands otherwise his flame will interfere causing control issues but this is just specualtion vander can give a good answer for this
Why I think Maria, the MC's daughter with Karla, will probably have more control over her magic than the MC when she grows up:
1: she got to have the fire ceremony before her power manifested fully, something the MC didn't get to have
2: The MC's first use of his power was killing his entire human side of the family, and he had to overcome his psychological barriers before he could even start training, we see Maria begin her training without that trauma at the end of the current update.
3: Magical talent seems to be somewhat genetic, and Karla is an enchantress, a specific branch of magic that seems to require a LOT of precision. If Maria inherits magic skills from both sides then she is going to inherit a type of magic that has a lot of power but not much control from us, and a type of magic with a lot of control but not much power from her mom.
I dunno it just seems obvious that Maria, and all the kids the MC is going to have in the coming years, are going to have a lot more control than he did. whether that comes at the expense of magical power is unknown.