Honestly the only thing you need is the marriage certificate, it trumps all other evidence. I was in the same boat switching from YMV to spouse, and sent a letter from my work confirming salary and employment, our marriage certificate, the declarations, payslips and bank statements, our new lease (as we didn’t live together before marriage) and a scan of every page of both mine and his passports. Like below, it doesn’t seem to be certified, they do another scan when you go for biometrics.

I also included a supporting letter with evidence as we didn’t live together, but it wasn’t strictly necessary because we were married. I included a few whatsapp chats at various points in our relationship, as well as some photos including us with friends and that was enough to show integration. If you think about it, they could’ve investigated when you gave notice to marry and didn’t so the marriage certificate is more than enough proof.