I agree with Kaitlan. I get that she wasn't there previously but we all deserve a chance. You're in a pretty safe position. If it doesn't go well you go back to the norm. I'd be open to it. Gracious during the awkwardness and if anything negative happens, get your husband to put the boundary in and if she doesn't abide by it then again, you can always go back to what it was Imagine if it ended up being amazing.
Impossible to know and I'd be suspicious, too, but it's worth having a conversation with her. Even if it's that the ultimate motivator is related to your baby, she may have realised that the best grandmother/grandchild relationships exist where the grandmother has a good relationship with their DIL. It would be best for everyone (you included) if you all got on well, though I can entirely appreciate why she's not flavour of the month. If it was me, I would keep the text response brief but polite and open, and then have a sit down conversation with her during which I would lay out all my thoughts and feelings on the table. I would certainly say that, while I wanted a good relationship with her, her previous lack of support during the most difficult of times had taken away that natural desire to want to be close to her. From there, I would say what I thought I needed to make the relationship work.
I agree that it’s definitely worth trying, but I would be cautious. I would also be open with your partner that while you appreciate this, and you want the relationship to improve, this does not change what has happened. And you in no way can just handle this switch while also navigating postpartum. That you want to have firm boundaries and establish with those are going to be and hope things can improve and get better- but she doesn’t get to decide the timeline.
@Kaitlan good points! It’s just hard for me to want to have a relationship when she wasn’t there for me in the past. I’m willing to try, but I think she realizes too that she has hurt me with things she has said and done and that it’s not just gonna be this overnight switch.