The thing is she has no red flags like you mentioned previously. Good family (both parents), minimal partying, dislikes drugs, minimal travel, no piercings, feminine and submissive etcFor sure. But my take is that OP messed up the vetting, now time has passed since the honeymoon phase, things have gone downhill, and he's had to resort to unsavory methods to confirm his questions about his partner's history and relationship value.
You can still look for flaws critically and take time and effort to pick the 'least flawed' option. OP likely messed up by not doing more of this at the onset of the relationship. But that's a common mistake a ton of guys, my younger self included make: you get so enamored with the new puss that you overlook a bunch of things; next thing you know you're one year into a relationship with the wrong person.
I look in her phone because I had a gut feeling she was lying about something, that I didn’t totally know her. She had said I was the only person she slept with since we initially met in feb 2018 - that’s an outright lie and I told her to stop capping but she insisted. Then I wondered what else has she been lying about, I saw some of the names on her bang list in WhatsApp and Instagram msgs from 2018 and 2019 - yeah your boy went full detective mode, none of the names are her friends on any socials.
The whole thing made me feel like a *****, there’s a reason I’ve never gone through a girls phone in 33 years. I can’t stop thinking about the 2x mmf threesomes that’s garbage behaviour lol.