If you're already using Specter in that project, I suspect it will give good performance here. But coming up with the right selector might be a tad tricky.

so far pure Clojure (no extra libraries for tree)

I will read about Specter

I'd start with something naive like this

(defn leaf?
  [x]
  (and (map? x) (not (some coll? (vals x)))))

(defn leaves
  [x]
  (cond
    (leaf? x) [x]
    (map? x) (mapcat leaves (vals x))
    (vector? x) (mapcat leaves x)))

(leaves {:a [{:a 1} {:b 2}]
         :b [{:a 1} {:b [{:a 1}]}]})
;; => ({:a 1} {:b 2} {:a 1} {:a 1})

you could then optimize it a bit further by only checking for map? and vals once

(defn leaves
  [x]
  (cond
    (map? x) (or (seq (mapcat leaves (vals x))) [x])
    (vector? x) (mapcat leaves x)))

I'd start there and then start getting into specter or other weird stuff only if that was too slow

https://clojuredocs.org/clojure.core/tree-seq

I'll second using Specter. It's awesome. Otherwise look into tree-seq, I suppose.