Good to see. I don't think we truly realise how deeply draining it can be in a BPD relationship until we start getting a taste of normality again and see the vast difference. To simply go out with friends and have a totally drama-free time without having to be on guard and walk on eggshells is such a relief.
You'll need to hold very firm boundaries when she inevitably splits from the woman she's seeing and tries to start the chaos again. She might not mind the see-sawing back and forth - that's BPD for you - but you need to put yourself first and not lose the normality that you've gained. Onwards and upwards.
What you initially feared (her seeking a GF) has turned out to be a reprieve for you. I agree- don't expect it to last. IMHO, this is one more external "seeking solutions" for her emotional distress and she's in the early "highs" of a new relationship. But you know the outcome this pattern.
That she isn't helping at home isn't really much of a change. Seems like a small trade off for the time to yourself.
This probably doesn't need to be said- but don't reciprocate by you also finding someone. That would be chaos and drama. Use the emotional space to reconnect with yourself and your kids. Also keep in mind that with BPD the "rules" apply to you but not to them. She would perceive it as a huge betrayal, even if she's doing it herself.
Wife with GF is an unconventional solution (and not likely to last) but it has given you a break from the chaos. It's a possible "exit" for you as it could be a deal breaker but, IMHO, only if you were to take action, and you'd have to decide on that if it were to happen.
Your wife now has what she imagines is the best of both worlds- your financial support, a family, a GF on the side, and no responsibilities at home. She would have no incentive to change this.