Yes, the problem is unfortunately not just the last/first step but the entire staircase's step height...
You can't really see how high the step height normally is in the staircase.
From what I see, I would estimate it to be 16cm, which makes the last step 24cm. If you were to change the last step and build an extra step, you would have three steps at 13.3 cm compared to the remaining 16.
I would decide not to change the staircase and instead test for a week with a temporarily built step of 12cm placed in front of the staircase. Then make an assessment of whether it became more comfortable or worse.
There is a "myth" that you trip in staircases where the steps vary by more than a few percent in step height, but it is very subjective.
You have the choice, as I see it, between how many steps should differ from the normal step height and what will be most comfortable. If the first step is now too high to be a single step, make it into two...
Ended up being long as usual.