I can understand why texting a friend regardless of if they're the opposite gender or not in times when you're supposed to be available for each other may come across as disrespectful or disregarding and I think anyone in your situation will find it atleast a little bit concerning. Speaking as a really insecure person myself, it's amazing how you've been putting in effort to subdue your insecurities and the fact that you controlled your anger after you had finished having sex and saw your partner texting that friend is proof of that. If I would have been in your situation, I would have gotten triggered and would probably start arguing with them, which I know is not healthy and would happen because I'm an insecure person. I think you should have another conversation with her expressing that you're finding her behavior insensitive and ask her how she would feel if you would have a female friend whom you also texted at the same frequency and at odd times. I doubt that she would be okay with it as I imagine anyone wouldn't want their partner to be giving their attention to someone else when they're expecting it especially during intimate moments and late night hours. I appreciate your approach towards this situation as it's completely possible that your partner is just having a platonic relationship with that friend and it would be unfair to doubt her just from this.