...
I’m open to being friends if it’s healthy for both of us. But I am curious if others have successfully come back from the devaluation and silence NC stage and engaged in a healthy boundary friendship? And also any tips on whether complete no contact while he is not engaging or the occasional check in over the weeks or months?
The typical BPD-ex-friend situation I've read here is that the pwBPD will test those boundaries in unhealthy ways. Either they'll be constantly trying to get back together, or they'll more or less ignore the other person (until they need or want something from them), but they're not going to be friends according to the dictionary definition of the word. They're not going to be there for you.
I think you're looking at this more as a game, with some sort of artificial rules of behavior, when really, you need to consider what you want and go do that. Are you hoping to get back together with him? It seems like he communicated a pretty clear "no" to you about that.
Do you really want to talk to him regularly, even if he's not there as a romantic partner for you? Then try that, and decide whether it's working or not.
Remember: boundaries are for you; you need to decide what you want, and what you will and won't tolerate. It's up to you to decide whether you're getting that from him, or not, and if not, when to end it.