You “know he has other female friends that the wife seems fine with”, but she had enough of a problem with your relationship with her husband that she tried to contact you directly. As he knows she’s been upset about your friendship for a while, it sounds like she’s tried talking to him, but he’s prioritising his friendship with you over his marriage. That doesn’t sound particularly platonic (or good for his relationship) from his side, does it?
As other posters have asked, many times - why didn’t you answer? If the partner of one of my friends called me I’d pick up, or at least send a message when I saw they’d called - there’s no reason not to. The fact that you ignored the call and spoke to your “fabulous, kind”, funny, colleague who you have such a ‘rare’ connection with instead is quite revealing. Personally I’d have guessed his wife was arranging a surprise party. You clearly guessed something quite different.
I have confided in a friend and she thinks I am playing with fire.
So…your colleagues wife has seen some of the messages and she’s not happy. You showed your friend and she thinks you’re playing with fire. You can even “understand why [colleague’s wife] could have been upset from a few of them”.
You know you’ve crossed a line.
“We don’t talk about feelings or anything personal” and “We honestly don’t have deep conversations” but:
I think he wants to talk with me because we are good friends and he could really do with a laugh right now. He’s had a difficult year (I won’t go into details as it would be outing)
You know enough details of his difficult year that it’d be outing to post them…but you don’t talk about anything personal? How does that work?
Look OP, if this friendship consists of only “jokey banter” and nothing deeper than that, you’re not “good friends” at all. It’s not some kind of rare, special connection where you’re lucky to know each other - you’re just colleagues who have a laugh. You should be able to reign in the evening and weekend chats quite easily, if you don’t want to see him stupidly blow up his marriage over a superficial friendship with a colleague.
If, on the other hand, you do share personal details, offer each other support, connect over just how much you have in common (musing about how it’s so rare to find someone like that 🥰), buy each other thoughtful gifts ‘just because’, send each other messages that even you acknowledge could understandably upset his wife, and you can’t bear the thought of having to reduce your out of work contact - you’re having an emotional affair, even if (somehow) you don’t realise it.
Am I being naive?
No. You’re lying to yourself though.