Thank you for your feedback. I appreciate that the restriction on carry capacity might not be everyone's cup of tea.
Just going back to your point about a low level magic-user only being able to carry 2 or 3 spells - this is effectively similar to how many spells a magic user has access to in, e.g. OSE. They have only a single spell at level 1, 2 at level 2, 3 at level 3.. etc. Whilst RAW a FORGE magic-user could potentially have 3 runestones at level 1. Yes this would be at the expense of other gear, but the option is still available to them.
Whilst I do agree that the rate at which you gain inventory slots as a magic-user increases more slowly than the max number of spells able to be memorised in OSE, the earlier availability of higher circle spells (e.g. 6th circle at level 6 instead of level 11) swings the balance quite considerably back in the magic-users favour.
In any case, as I mentioned previously, please feel free to increase carry capacity to your preference. For example, using (STR check bonus + 15) inventory slots will give you more wiggle room at lower levels without breaking anything.
I hope that helps.