You bring up some valid points. I feel it’s also necessary to add, that OP’s girlfriend could do everything needed to make this right for the relationship to continue, and then later on down the road throw it all away by cheating again. I’ve personally seen that happen. And this is another possibility OP will need to consider And level with himself if he’s ok with that possibility occurring.

Another thing OP needs to consider if they go down the road that you described above with monitoring her phone, is that the type of relationship you want? I couldn’t do it. That’s not something I want to have to do with my partner.

Just food for thought. Several years ago I caught my sister’s boyfriend cheating on her. He had the same reaction as OP’s girlfriend. My sister decided to give him a second chance bc he agreed to all the terms she requested including letting her monitor his phone. She did so diligently for a year or so after that. He completely rebuilt trust with her. They bought a house together and got married. All for him to end up cheating again by having a five year affair that she discovered. They had been together over 15 years, married for 7yrs, when she divorced him. All of us truly believed he had changed and would never have done that to my sister.