Mine's this-ish (nushell, but easily bashified or pwshd) for finding all merged, including squashed:

    let t = "origin/dev"; git for-each-ref refs/heads/ --format="%(refname:short)" | lines | where {|b| $b !~ 'dev' and (git merge-tree --write-tree $t $b | lines | first) == (git rev-parse $"($t)^{tree}") }

Pipe right to deletion if brave, or to a choice-thingy if prudent :)