Maybe there's a way to secure a small bracket with a nut and bolt to keep the adjuster in place. Going past the highest notch was just an idea, I personally haven't tried it but it seemed like it would work. The highest level is still scalping your lawn???
I have a very deep texas san augustine lawn. The tall skinny Snapper wheels sink into the lawn leaving the deck deep in the grass.
On the highest deck setting the mower is cutting 2/3 to 3/4 of the grass blade.(instead of 1/3) Cutting this much of the grass also overwhems the mulching blade so I had to put the side discharge plastic back on the mower.
Picture of grass taken 2 days after mowing.
Since I mowed 2 days ago and struggled with it cutting too deep, I have raised it beyond the last notch and tie-wrapped it in place.