A lot of people have asked the same question over the years, and by now there are some good answers.

For example many lisps implement SRFI 105: Curly Infix https://srfi.schemers.org/srfi-105/ which lets you write {a * b + c / d} instead of (+ (* a b) (/ c d)).

Afaik Fennel doesn't have this, but that's fine because it has macros!

  (macro infix [expr]
    (let [stack []
        ops {
          "+" (fn [a b] (+ a b))
          "-" (fn [a b] (- a b))
          "*" (fn [a b] (* a b))
          "/" (fn [a b] (/ a b))}]
      (fn [item]
        (if (number? item)
          (table.insert stack item)
          (let [op (get ops item)
              b (table.remove stack -1)
              a (table.remove stack -1)]
          (table.insert stack (op a b)))))
      (each [_ v (ipairs expr)]
        (infix v))
      (first stack)))

And now you can do:

    (infix [1 + 2 * 3 - 4 / 5])
    ; => 5.2