Hwei has 4 cooldowns (Q/W/E/R) but each base ability can select from 1 of 3 options, so he has a normal mage kit's shape of cooldowns just with more options.
Each of the basic abilities is themed for art and gameplay, so Q is damage, W is utility and E is control. Each time you cast, you're picking the best tool for the job of that type of output.
Every ability was designed with a niche that it's best at when compared against its two share counterparts. Some games you might find more reliance on some abilities and less on others, and that's okay. Other games it might flip and you're using the reprioritized ones from last game most often.
If we need to balance cadence or sustain, we can touch base cooldowns and mana costs. If we need to address specific problem spells we can do that on the particular spell by buffing or nerfing its outputs without touching the others. But because they all have a niche use case, no single spell should ever become worthless.
Hwei's mana sustain is countered by high mana costs. One of the most valuable skills a player looking to master Hwei needs is when to use WE to refund mana, finding units to hit 3 times to do it, and how to play safely while his shields and movement speed are on cooldown because of it.