Today, I somehow encountered the concept of "adjustable-legs" for > benches.
IMHO, bad encounter.
If you need both a "high" and a "low" bench, then build both a "high" and a "low" bench.
After the bench gets built and a couple of years of "Maybe I'll use it someday" crap gets piled on, around, and under the bench, it will take a couple of days just to get at those "adjustment" bolts.
And don't forget the fork lift truck you'll need to lift the bench since it now weighs at least double what it did when you built it.
Have fun.
Lew