There is hope!
Yes, there is hope - sometimes there is more hope than other times. You sound very very committed to your marriage. That is good, because there will be times that will be wonderful and there are times that will be not so good.
I think I am not well versed in the acronyms used here - and they tend to be different from place to place where people talk. Excuse me, but I am not sure what "pwBPD" means.
Regardless, DBT can be very effective. But it helps if the person with BPD is very receptive to it. Some are, some aren't - and it sounds like your husband is in the "appropriate" category. That says a lot about his counselor - they sound like they are very good at this... I've known people who had a SO with BPD and the BPD goes to a therapist and is immediately diagnosed and told to enter a DBT program. THe BPD freaks and dumps the therapist and it is actually negative progress. But it sounds like your husband is handling it well and has someone to help him that can handle it well.
Please take time for yourself. Yes, that is cliche - but it is true. It's very easy to get wrapped up in their life over the years so much that you lose yours. It isn't good for you, and it isn't good for him. But I will admit that it is tricky walking that tightrope of what is appropriate to say and not. When people go the "stop walking on eggshells" route (not that I disagree with the concept), it is so easy to try to work with the person, that you lose perspective of how a relationship can be or should be. It's hard to be "on guard" all the time, watching what you say or do all the time. Boy, it's very hard to always communicate in that "perfect" way. Sometimes you trip up out of exhaustion.
Well, I'm very happy for you that your husband is getting therapy. I've been out of communication for a whole about these things, but not too long ago, BPD SO's that went to therapy willingly are rare. So yeah - you've got a good start. I hope it goes well.