So while this has resulted in a bit of showing off knowledge for other purposes, the simple answer is that it cannot be done (as of July 2023).
If only Quickbase had a Formula that accepted a "Max" argument as a delimiter-separated string instead of multiple arguments. There is not way to turn a string such as "1; 2; 4; 10" into multiple argument for the Max function to process.
There is a way to pull this off in a Pipeline, but no way to run a Query for max and no way to determine max in a Formula Field.
If you're working with a small or predictable data set, you can cheat this with the following snippet. You can extend it to support as many potential input values as you want.
I wouldn't blindly condone using this, but if it works, it works.
//debug = 9;10;11, or your query result converted to string
var number a = ToNumber(Part([debug],1,";"));
var number b = ToNumber(Part([debug],2,";"));
var number c = ToNumber(Part([debug],3,";"));
Max($a, $b, $c)
//result = 11
