Affairs are a funny one on MN. People instantly say LTB, when it is a lot more complicated than that. For lots of people, LTB means swapping one shit show for another.
I used to live in Asia and marriage for a lot of people there is a lot more of a practical arrangement than western people go into. Westerners put a lot more emphasis on love and/or lust, which a marriage counsellor told me lasts 18 months - 2 years.
My DH has had 2 wobbles in our marriage and wanted to leave, but didn't in the end. The first was pre marriage, which would have been OK as I was working in a good job, but the second was with young DC. That would have left me in a very vulnerable position. In the end he didn't, but it made me change a lot of things. That included my work, my pension, our house, our savings and investments. He may have another wobble in the future, as he has form for it, but next time I am ready for him. My DC will be older, and not need looking after, all my ducks are in order.
It seems like you are covering yourself, but in your shoes, I'd be doing more. You are happy with the status quo, but how are you going to feel if he one day turns around and says hey the DC are older now, I am leaving for OW. You will feel a right mug. I'd be making sure I was 110% sured up, and in a great position for any bombshell. I'd also be getting evidence of his long-term affair.
Finally, why should he have all the fun. Find yourself a bit on the side, guilt free. He has broken your marriage contract so you are free to so whatever you like.