FWIW, here's how some numbers work out:
When casting the highest level spell you can, you have about a 1 in 3 chance of losing it.
When casting the highest level spell you can later, you have about a 55% chance (on average) of losing it. (Which not surprisingly is the same as rolling the 1 in 3 with disadvantage.)
You get to a point, though, were the chances of failing drops off so much I wonder if it is even worth the bother of rolling?
This is why we decided to abandon the idea of making checks. Originally, we were going to use these checks to successfully cast the spell, not to resist strain, but the numbers work the same.
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What I might do is just make it that if you are casting your maximum level spell, roll the d6 to avoid losing it. On a 1-2 it is lost. After the first casting, the roll is made with disadvantage. This would represent the strain of casting your most powerful spells (or at those spell levels). Just a thought.