1. I don't care for
Max regardless, but to me it's fine as a full name. I wouldn't bother with a longer name just so he can have more options.
2. I'd choose
Maxwell out of the ones you listed, just because it's the least pretentious of them all. Lol, I can't imagine introducing myself as
Maximilian or
Maximilianus. : P
3. Out of your longer list, I really like
Maxen. It manages to be usable, trendy, and yet rather unusual, an interesting combination. I'll also suggest
Maxim (though I wouldn't recommend using it) and
Maddox.
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“Violence, in truth, recoils upon the violent, and the schemer falls into the pit he has dug for another." -
Sherlock Holmes,
The Speckled BandBTN's Resident Historian