He has cheated on you by sexting. She is a work colleague, so I assume he sees her daily. It is possible that they have been physical. They could easily be continuing their affair.
Did you impose a consequence for his betrayal such as sending him away for a while? This would show him that you mean business and would give him an inkling of how the loss of you would feel.
Therapists advise that it can take up to 2 years to regain trust after an affair. You’ve barely scratched the surface. For affair recovery, they recommend full transparency with all devices, bank statements, phone bills, credit card statements, etc.. You should not have to sneak a look at his phone.
To restore trust, your partner needs to show great remorse and empathy by consciously reassuring you without being prompted. He must patiently answer your questions at all times and be willing to take your anger and hurt without snapping.
Completely cutting contact with the affair partner is advised.
Has he been open to seeking individual counseling to investigate his weak boundaries and sense of entitlement to pursue an illicit ego boost, and to learn how to safeguard his fidelity?
This man who abused your trust is not showing much remorse. He has wounded you, yet has built no real structure to help you heal. He appears disengaged right before a trip away, even though you have been clear about your (understandable) need for reassurance.
Personally, I would walk away.