Here are my thoughts!! (Get ready for a TLDR quote after)

Call it based off D&D, because it isn't D&D. That gives you a LOT of freedom!
Stuff the rules, and do it as you think makes more sense!

1) Levelling and Level Scaling:
Start the characters with a higher base hit points. Make increase in hit points per level less, and level it off after a certain level (say level 9).

Allow creatures to "spawn" at a level related to the players level within a limited range (like skyrim and fallout4). So Goblins will be level 1 to 8, but you wouldn't find a Goblin higher than level 8, and Level 8 Goblins would be very rare.

At higher levels, spawn more creatures.
Consider different enemies to spawn at higher levels.

Example:
Level: Enemies Spawned:
1 3 Level 1 Goblins
5 5 Level 1 Goblins, and 2 Level 3, and 1 level 5 Goblin
10 3 x 8HD Ogres
20 5 x 10HD Ogres, and 1 Ogre Mage (HD 15)

With the reduction in hit points at higher levels, you find that you are not as "OP" at the higher levels!! AND the reason to increase in levels is to get more skills!

Also take off the level cap! Make levelling from 1 to 9, as it is now, but from levels 9 to 15, make it harder, and from level 15 onwards, make it extremely hard and long to get to the next level!

2) Attributes
The attributes are what defines a character!
I would let people increase their attributes with 1 point every 2 levels, but only let them increase their attributes say 3 points from their starting point. This means that what they are born with, defines the sort of character they can develop into!
Here is a thought: Increase the size scale of the character model based upon their strength!

Question: Do attribute bonuses from gear stack? - I wouldn't let them stack!

3) Skills
I actually think the skills are well done! I think it is a good way to depict it. As for the skills themselves, I have not got enough experience with the game to comment. The fact that you can come into a new skill later on, and then develop that skill further is a great idea!
Personally I think the more skill Options the better, and include more passive skills as well.

4) Magic Schools and Cooldowns
Again I think this is reasonably well done!
A suggestion I could make, is that some spells should take longer to cast than others (the big spells! Things like fireball etc). These can then be interruptible!

I also think that you could make it that you can cast spells at any time, but chance of miscasting the spell is greater if the soft cap is not reached! (ie. The concentration to cast that spell is significant and you need a break to get the mental energy to cast it again)
And have a slight chance (that varies on how soon between casts) for a miscast of the spell to have a disastrous effect – have like 3 things such as a) a small explosion of fire damage that injures the caster; b) a large fireball explosion centred on the caster ; c) The caster is stunned for a variable time!

This makes the idea of having cool-downs more explainable / immersible!! With the chance for pushing the boundaries for some mages with bad results!

I would also suggest adding more passive bonuses to certain types of damage to make it possible to specialise in certain types of damage or spells schools: ie. More passives on the trees to make mages (or casters) more powerful at later levels (and possibly weaker at early levels)!

Another idea would be to include at the end of the skill trees some passives that apply to that skill tree! A reward based upon focussing on a skill tree!

5) Gear
Don’t we live in a materialist age?!?
I think gear benefits should be also reduced / scaled down!
The gear to level benefits ratio shouldn’t be too high!
So a fully maxed out geared level 15 should be equivalent in power to say a level 20 with unenchanted gear. (I just pulled that out of my head.... but you understand what I am saying!)
Ie.... pull down the materialism, and make skill choices, and the way that people play more important than gear!
That was one big mistake in my opinion with WoW (along with poor level scalling that needed continuing increasing in level caps!)

These are just some thoughts and ideas! I am hoping some of this may inspire some creative, new and fun gameplay!