First up, I'm going to declare a bias: my husband never stopped using prostitutes - I could never find quite the evidence I "needed" - although I found plenty after I filed for divorce and he got careless 
Just because my husband didn't stop acting on his 'kink' doesn't mean you husband hasn't. I know that. But here's my opinion...
Sometimes, you're into something sexually, your new partner isn't, so you leave it behind. But some kinks are more compulsive than others - and easy to compartmentalise. Dogging if you just watch? Well, it's not cheating on you, is it? (he could say) I expect (and other posters are free to call me biased and point out that dogging isn't prostitution) that dogging is more on the compulsive and difficult to walk away from side.
You allude to there being other things. There are the hook up site profiles, undeleted. The fact he still had a car full of tissues and glo sticks AFTER he met you. And the trousers under the carpet? Well that's the most damning thing. Because that DIDN'T happen by accident.
What I think people who haven't been here don't perhaps realise, is how fucking soul destroying it is to have 20 small suspicious things, no single one is proof, no single one is big enough to leave over when you have a child. Any single one can be argued away. Even the one's that can't really - come on, who EVER on here accidentally lost a spare pair of trousers under their boot carpet?
All these things added together creates the body of evidence that destroys the trust and destroys you. But if you describe one to a friend, to a counsellor, to yourself... It just doesn't seem enough. Not to end a marriage over.
It is my guess OP, that in 8 years you have amassed quite a list of things. Photos of an ex... wouldn't bother me. But you also allude to "other bits I wasn't happy with".
I think that you end up with 1 or 2 things that you just can't explain away, to yourself. In my case, I found searches for local escorts on my husband's office laptop, only ever accessed in a shared office (family business, small office for all my previous digging I found these searches accidentally
). His previous explanation was it was only porn. Well - that just didn't fit with a search done at work where he frankly would never have been knocking one out. Hard to explain to those who haven't had the 1000 cuts. But you just can't explain away some things. I'm feeling terribly sad for you, because trousers under the boot carpet falls into that.
The thing is, from the outside, there's no proof there still. It sounds ridiculous "I left my husband because there was a pair of trousers with the spare wheel". It's so vicious, that it makes you look paranoid.
Me? I left my husband because there were lots of deleted text messages to unsaved numbers on a phone a year old. Sounds like not enough to break up a 4yo's home, huh?
I was fortunate to get my evidence later - saw all his adultwork bookings. Including the ones he continued to make after getting a new girlfriend.
Because it's compulsive. (I'm not excusing it, saying that!)
OP, I can't tell you what to do. I stayed too long in my own marriage. But I can't assume that your husband is like mine.
All I can say is, I know how hard it is when it looks paranoid and ridiculous to end it over an oddly placed pair of trousers. And I also know how it is never just the one oddly placed pair of trousers.
Much love and strength to you. xxx