I do actually believe she doesn't want any more. Had it been truly up to Sue, I think she would have stopped at Archie. She made such a big thing of it at the time that 20 is her limit.
Noel will keep chipping away at her until she caves though, for the vlog content and money it'll generate. I really get the impression that's what happened the last 2 times. Sue is genuinely excited when she's pregnant, as she's receiving the attention she craves, but her body language these days suggests she's truly disinterested when she has given birth, even in the newborn days now (which she use to love). She just seems tired and bored by it all.
So I can actually believe actively trying again wouldn't be her choice, but I don't rule out her being talked into it yet again. If she conceives, it'll be billed as a happy accident (again, ala Heidie) If she doesn't, due to age, it won't be for the want of trying on Noel's part. Sadly we all know what a hold he has over her. He's been dominated her since she was about 12 and she just doesn't know how to say no.