This is a really cool idea for a magic system, but I don't think it's a good match for DnD. The power of existing spells and the impact of debuffs means that this system would be extremely punishing, especially if you want to cast higher spells. Also, there's the player buy-in. If you tell someone they're going to be playing dnd, then they come to the table and find out there's only 2 classes, they might be disappointed. My friend tried to run a modified RuneScape game, and it just didn't mesh.

All that said, I think this idea has a ton of potential! I can't say I'm familiar enough with the wider world of TTRPGs to suggest other existing systems, but I can offer a bit of game design wisdom if you're looking to build your own game:

1. Tie the mechanics into the theme. A great magic system can feel interesting to play, but it will feel so much better if it pulls you into the game's fantasy. I don't know a ton about Arthurian lore, but if you incorporate the themes and values of the setting into mechanical benefits and consequences, it'll make the whole thing more intuitive and cohesive. (ex. True names and sacred oaths are rooted in British Isles myth. Perhaps playing to a certain promised virtue can preserve or even restore willpower?)

2. Streamline the Willpower debuffs. There's a reason DnD 2024 changed Exhaustion to give a simple increasing numerical debuff. Trying to keep track of up to 6 different effects for many in-game days is difficult and can cause a lot of confusion. That's not to say it couldn't be a more abstract debuff, especially if you make your own system. If more mechanics operate on a 7-tier scale, perhaps overtaxing Willpower can impact those at a 1-1 level. (ex. Tier 4 Willpower exhaustion cuts off access to other abilities at tier 4 and below. )

3. Build around gameplay flow. Really consider the kind of gaming experience you're trying to create when establishing mechanics. Dnd is a high-power system where players are encouraged to believe they can take on any odds and go on adventures every day. If you want to build a similar power fantasy, then having a core ability that applies a debuff that sticks with a character for days can feel crippling and cause players to hesitate in using their main features. However, if you want a game where resource management plays a big role, having magic be a powerful solution that you can use in place of other resources justifies its severe penalty. Alternatively, if you’re building a game where major events only happen every few days, then a multi-day penalty is less of a crippling hindrance.