YABU.
Your DP's ExWife presumably took his surname when they married. She now shares that name with the kids and I can see completely why she'd not change that.
You said "She does contact him a lot to get him to 'do stuff' like fix shelves, a bit less so now." This is in the family home she shares with their children, I presume, so that seems fair.
Do you mean 'his in laws' or do you mean her former in laws, her children's grandparents, "...cards for their birthdays and Xmas always signed 'from the X surname family'." But she is the surname family. They created a family together, and because one or both of them chose to end the marriage it doesn't alter the fact that the family continues.
"Maybe I should just get over myself and not think about it." Or maybe you should work out if this bothers you so much you don't want to be with him? Because being with him means accepting he was married before, he has an ex wife and he has kids and they all have his name, and he sometimes needs to help the family out.
And she is still in your DPs 'family unit' but the fact you feel 'more than me' suggests you have some insecurities about the relationship and I think those are the things you need to address.
Re 2We will marry at some point but the thought of taking his surname also makes me feel weird, like I'd be wife number 2!" You wil be his second wife, not wife no 2, wife number 2 suggests you are in an old movie about ancient China where men married about four times! You are not. And you do not need to take his name unless you wish to.f you feel insecure about your place in his affections, you need to address that, and certainly before you marry or have a family together.
Did he leave her for you or for any other woman? Could that be worrying you? Does he treat her well and with respect despite being divorced?
If he treats her well and she still has a respect for him this is a good thing and shows him a good light, f he did not have anything good to say about her or vice versa I'd say that would be cause for concern, potentially.
Address your concerns, but not about her, about him and about your place with him in the life you are creating together now, not the life he created in the past.