The problems with these spells are that they are 1) basically useless when you buy the next level up, and 2) there are restrictions on what you can cast them on.
1) I am going to use sun school as an example. To get gargantuan I had to spend a bunch of training points on the lesser spells to get there and now those are useless where as in the wizard schools those other spells that you purchase are still useful in various situations. It’s basically a waste of training points. It would be one thing if training points were more available and there wasn’t a limit on them, but there is, and when you use them you can’t get them back. I feel like we should be able to “stack” these spells OR unlearn them to get the training points back.
2) they can only be used on our regular deck spells. I would like to be able to use these on everything, my permanent spells, treasure cards, and cards given by my equipment. Truthfully I would also like to be able to cast it on teammates spells, for example if I see someone is about to use storm lord I can boost it up, I would like to see that added but lose my round on it, I feel like that would be fair.
On Sun, I don't think it's an issue. I see it as more powerful spells having a higher cost. If I want to learn Feint from death school, I need to learn Dark Sprite first, so Sun is on parity with the seven main schools. Stacking them would annihilate any balance the game would have after Zafaria or so by letting you enchant the spell multiple times for massive amounts of damage beyond the base. Star doesn't have the issue you complained about at all. The Star school auras are diverse and any Star spell isn't going to cost you more than a few points. There are also very few "strictly better" auras, and a large level gap between the weaker and stronger ones. Moon is just a garbage fire. I wouldn't bother with it if I got them for free. Polymorphs need a major rework to be viable anywhere.
I don't have an issue with the second point. It is pretty weird that I can't enchant an item card.
On Sun, I don't think it's an issue. I see it as more powerful spells having a higher cost. If I want to learn Feint from death school, I need to learn Dark Sprite first, so Sun is on parity with the seven main schools. Stacking them would annihilate any balance the game would have after Zafaria or so by letting you enchant the spell multiple times for massive amounts of damage beyond the base. Star doesn't have the issue you complained about at all. The Star school auras are diverse and any Star spell isn't going to cost you more than a few points. There are also very few "strictly better" auras, and a large level gap between the weaker and stronger ones. Moon is just a garbage fire. I wouldn't bother with it if I got them for free. Polymorphs need a major rework to be viable anywhere.
I don't have an issue with the second point. It is pretty weird that I can't enchant an item card.
Ok true but at least dark sprite still has its use sometimes. I will literally never use the lesser sun school spells once I get the next one up.
While reading your concern, I had the idea of keeping the current training point expenditure, but instead of getting a new spell, it just upgrades the existing one.
So instead of having 5 different damage enchants spells in your deck, you'd only have the most powerful version you have learned.
On a similar note, I hate that Radical isn't available until level 110. trying to use Unicorn after level 40 is pointless and the next aoe heal learnable via normal questing is a shadow spell.
The problems with these spells are that they are 1) basically useless when you buy the next level up, and 2) there are restrictions on what you can cast them on. 
