I played a few runs with this character, and I really really like the concept! However, the current execution unfortunately doesn't feel like it does the concept much justice. Hopefully since it's a work in progress you're looking for feedback - if not, feel free to ignore my suggestions.

Since the concept is modular spellcasting, I think the cards should push home that combining effects yields better results than the sum of the parts. I have a few thoughts on that.

1. Arcane Barrier should definitely not be in the starting deck. The concept of the card is fine - strong spell effect with a drawback in the form of negative Spell Modifier - and it would have its place in the mod design if you chose to keep it, but I think that place isn't the starter deck. Due to its drawback, it's more effective play it and cast the spell before playing it and casting again, so the (as now) only modular spell component you start with performs poorly when you try to make a compound spell. This seems like it should be against the design you'd want. I'd suggest maybe replacing it with Freeze, which fits a similar role with a more vanilla feel, or even creating a new card for the starting deck. If you want it to be a Block card, I'd suggest something on the order of Spell Effect: Gain 4 block. Spell Modifier: +25%. That way it's as good as a Defend with Retain if you use it once, but if you put 2 on a spell before casting you get rewarded with 12 Block instead of 10, and it could synergize with other Spell Effects you pick up, reinforcing that combining modular elements is how the character functions.

2. Speaking of the starter deck: it could be better outside of the above concern. Since the character is about the modular spells, they should probably start with more than 1 card in that theme. Ideally several that shine when combined with themselves, each other, and other cards you'd get later. I'd suggest a card slot for a Thunder effect of some sort, as it's more proactive than Block, and gains more when stacked with itself. Maybe 4 Strikes, 4 Defends, 1 "Spell Effect: Deal 6 damage.", and 1 Spell Effect Block card as above?

3. Most of the modular effects just add to each other, but don't synergize much. I think that could be improved. For example, what if instead of a standalone attack, Ritual of Sacrifice was a card with "Spell Effect: Deal 7 damage. Spell Effect: If this spell kills a creature, heal... {as it is}"? Would give a similar effect but further build on the spell stacking feature. What about "SE: Deal 5 Damage. SE: Deal 5 damage to self. SE:Heal for 25% (or some tuned number) of unblocked damage dealt."? "SE: Deal 4 damage. SE: Ignores Block."? There's a lot of potential to reward players for building spells that are more than just (X+Y+Z)*N damage/block.

Anyway, that's my first-impressions ramble. I hope if you took the time to read it you found something helpful - I really do like the mod, and I'd like to see it be the best it can be!

Edit - initially was going to post as a comment but I was way over character count - I'm a bit of a windbag!