[name_m]Arthur[/name_m] is such a lovely, classic name!
[name_f][/name_f]From your list, [name_m]Caspar[/name_m] is probably my favourite as it feels like a great stylistic match. It has the same vintage charm as [name_m]Arthur[/name_m] while still feeling distinctive, and it doesn’t seem overly common. [name_m]The[/name_m] only possible drawback I can think of is that [name_m]Arthur[/name_m] and [name_m]Caspar[/name_m] have somewhat similar endings, but that may not be an issue depending on how you feel about it.
[name_f][/name_f][name_m]Maxwell[/name_m] is strong and familiar, and I like the nickname options (Max/Wells). It does feel a little more modern compared to [name_m]Arthur[/name_m], but if that contrast doesn’t bother you then it still works well.
[name_f][/name_f][name_m]Kit[/name_m] is really sweet, though to me it feels a bit nickname-like next to [name_m]Arthur[/name_m], though it could still be very cute!
[name_f][/name_f]If you’re open to other ideas with a similar feel, maybe [name_m]Hugo[/name_m], [name_m]Felix[/name_m], [name_m]Otto[/name_m], [name_m]Cyrus[/name_m], [name_m]Louis[/name_m], [name_m]Oscar[/name_m], [name_m]Jasper[/name_m], or [name_m]Rupert[/name_m] could work too.