This will be a slightly difficult question to answer, not because we don't know the answer, but because it's really up to the player.
First off, just a general note. In our minds, story/setting needs to come first. It's possible to work up a mechanically perfect system that would not fit the feel of Symbaroum at all. So the system needs to feel right, it needs to reflect the growing darkness and make people feel like they are gambling with their soul.
So on to the math:
At 1st level, a Mystic's Corruption Threshold is probably 7 (proficiency bonus of +2 x 2) + spellcasting ability modifier, which is likely +3). The cost for an unfavored 1st level spell is 1d4+1 temporary Corruption, the 'expected value' for a d4 is 2.5, so you can cast two 1st level spells, which is shockingly close to what a Wizard or a Cleric can do at that level. Of course, Mystics don't prepare their spells, everything they know they have available to them.
At 5th level, a Mystic's Corruption Threshold is probably 10 (pb +3, ASI at 4th level). To cast a 3rd level spell is an expected cost of 5.5, which means casting two of them means you'd be over your Corruption Threshold by 1. So that's the first part of the 'push your luck' peeking through. More on that in a minute. Of course, favored spells also can help tremendously, since they turn the expected value for that 3rd level spell into 3, instead of 5.5.
At 9th level, their Corruption Threshold might be 12 or 13. Two unfavored 5th level spells cost 15 temporary Corruption, so we're exceeding our Threshold again, by just a touch more. Of course favored spells are still around to help mitigate this cost.
THE FIRST PUSH-YOUR-LUCK MECHANIC:
The first, as I'm sure you've noticed, is that the Threshold is not a hard and fast limit. If this was a spell point system, you'd run out of points and couldn't cast anymore. Instead, you can keep going, but every time you add to your total and you're above the Threshold you have to roll the d20 and, if your result is under the difference between your current score and your Threshold, you're going to need to deal with consequences. This can allow you to dance over the line and hope your luck holds. Or you can completely nova some big bad but then you're going to be dealing with the consequences of that decision for likely some time.
WHERE ARE MY LOWER LEVEL SPELLS?
You've probably noticed that I was only paying attention to your highest level spell slots earlier. Of course, it's likely you're going to want to cast some other spells too. You'll make room for those by casting favored spells and takings rests on occasion. A short rest gets you your proficiency bonus back in temporary Corruption and a long rest gets you twice that. Those are free, but you can get even more back by paying. More on that in a moment.
At 1st level, a short rest gets you two points. Maybe enough to take a gamble on an unfavored cantrip or to cast a favored 1st level spell a couple more times.
At 5th level, you get 3 points. So an unfavored 1st level spell or favored spell of any level.
At 9th level, you get 4 points. Several favored low level spells or one or two unfavored spells.
Again, it's impossible to predict what the exact mix will be, because it really depends on the balance between favored and unfavored. It might be a little less than Wizard/Cleric but its still more flexible because you still have all your spells available to you.
THE SECOND PUSH-YOUR-LUCK MECHANIC
Notably, this one has you gambling with your life. In Ruins of Symbaroum, Hit Dice can be used to recover hit points or temporary Corruption. For either a short or long rest you can spend Hit Dice to recover HP or reduce temporary Corruption. You don't recover Hit Dice until you take an extended rest or a character's feature gives you a chance to do so. So, yes, a 1st level and a 3rd level (or 9th level and 11th level) character has the same Corruption Threshold and recovers the same amount of temporary Corruption on a short rest, but the one with more Hit Dice has a choice to gain more magical power or more hit points. That should always be an interesting decision.
In the future, we will talk more about new Approaches to the Mystic and how some of them approach managing temporary and permanent Corruption in completely different ways.