I’d probably just do a different middle – but adding a second one feels like a perfection solution if you want [name_m]Alexander[/name_m] in there.
I really like [name_m]Oak[/name_m] for you, along with [name_m]Heath[/name_m], [name_m]Leon[/name_m] and [name_m]Zeb[/name_m] too 
I really like [name_m]Grover[/name_m] [name_m]Benjamin[/name_m] too – and it breaks up the -er endings.
[name_m]Grover[/name_m] [name_m]Leander[/name_m] (Leon + [name_m]Alexander[/name_m]) did come to mind but might not work