I checked it out for curiosities sake, skimmed through some of it. I would say he's not really a doormat, but definitely on the passive side and definitely gives submissive vibes with some of the texts. I do agree it's kind of ironic the dev's name is stag considering the MC is clearly more of a cuckold, personality wise.
I feel like the phone style games only work well for actual NTR and not NTS. NTR with the phone only style games I can imagine would be intriguing for those who enjoy NTR, especially with the mystery element added to it. Whereas, with NTS, you are pretty much never with your girl. When the MC and her are together, it happens off screen and is only implied, since the game all takes place through text messages.
Definitely not for me, but I enjoyed checking it out, despite the passive cuck MC, the GF's teasing was pretty hot. I know some people out there, especially those into cuckold stuff, would really enjoy it.