My assorted thoughts:
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I prefer Max standalone to Maxwell - anyway but also think it’ll flow better than Maxwell ___mas (a little choppy?). Having said that maybe Max with a mas last name is also a little funny/repetitive sounding, not sure? But I find Max to be such an appealing name, and it’s a fab brother to Arthur.
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I love Caspar - I prefer the softer looking Casper but get some might want to use Caspar. Jasper is also a fave.
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Kit is another great choice but depending on your last name, I’d think about Christopher or Beckett nn Kit - eg if your last name starts with T, think Christopher flows better than Kit (ending in a T). But Kit alone is very spunky!
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Other names I like with Arthur and on their own terms: Sebastian, Felix, Quinlan, Harrison, Wolfe, Hugo, Hugh, Edmund, Julian, Tristan, Vincent, Tobias, Wesley & Westley.
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Like the names but wouldn’t choose Henry or Theodore as very common choices over the last decade, including as brothers to Arthur.