I believe it would benefit the op to separate the two missions of marksmanship and defense.
For the second, a shotgun or centerfire pistol or carbine are likely choices.
For the first, I concur with
@Singlemalt re a .22 revolver. It’ll teach all the gun-handling basics. I would suggest that a good air pistol would make for target training, although the remaining gun skills (carry, draw and aim, tactical reload etc.) would get neglected.
The .22, your protestations noted, is still marginally in the club as a defense round.