Somuch if this is all for real I would say speak to women's aid because he sounds abusive. These are the words from your OP that stand out at me....
'...so trivial compared to some threads on here'
How is this ever trivial?
'I recently found he's chatting online with women he's 'met' on porn sites, exchanging fake images and really explicit messages.'
So what's to stop him telling one of these 'women' (who might be a middle aged man called Frank, who is also exchanging some fake photos) about his lovely wife and daughter and where you all live!
'I know he watches porn, it doesn't particularly bother me'
Porn is really corrosive and offensive, I think it really can destroy marriages (IMHO). The women are treated very badly and the images affect how men see women.
"...it makes me feel shit and I find it hard to respect him"
Of course it does and of course you do, if he loved you, as he says, would this not bother him?
"I may sound delusional, but I have no concerns about him having an affair. It's all anonymous, it's not rooted in reality for him at all."
Yes, of course it sounds delusional!
My husband is the most loving and sweet guy imaginable but even he could have an affair, so if your other half is engaging in sex talk with strangers I'd say you are deluding yourself big time in thinking he will never take it further.
"He says he does it when he's bored, he knows it's stupid, etc etc."
How much time does he have to be bored with! It's pathetic. He could get a real hobby, take up a sport, spend time with the wife he loves and the child, or do anything other than message random people with sex talk.
"I don't believe he'll stop."
So can you put up with it? ( I would not!)
"DD sometimes uses his iPad to watch cbeebies, and twice she has got into his pictures app and found explicit pictures that he has downloaded in order to send in messages."
How shit for her.
"...promised he'd make sure it couldn't happen again, and now it has happened again."
How shit for her.
"I already struggle with the inequality in our relationship."
Yes, you are committed, he is having flights of fantasy who knows who.
"...our next round of IVF (which is now on hold)"
I had years of IVF. I know it is hard. But you know if you want another child, you could find someone to have a child with who will not treat you and the child like this? Who will respect you both.
"lately has repeatedly done things that compromise our safety (leaving keys in door, leaving applicances on all night / all day when we're out, forgetting to close front door at night) - and I mean repeatedly."
That's scary.
"When we argue about things like this he won't take it seriously"
How charming.
"he reacts like I'm hysterical"
Nice way for him to dismiss you totally. Gaslighting
"..if I push it he gets angry"
Angry what the fuck has he got to get angry about?
"he won't engage with me at all."
That sounds classically abusive.
"I'm so confused"
Call Women's aid, if you are for real they can explain to you why you are confused, because he has treated you so badly for so long you do not know which way is up.
You have said you are going to talk to him, do if you wish, but I'd talk to women's aid first, especially if you think he may become aggressive.
I do believe you but then again I am just worried, this is so extreme and your thoughts that it may be normal/trivial are very worrying.
www.womensaid.org.uk/
0808 2000 247
[hugs]