I get the feeling it's not really the frequency of visiting that bothers you, but the way that it's become a routine.
If every weekend doesn't suit you (and it wouldn't suit me!)then I don't think you should have to see them every weekend. But I think you (or ideally DH) should tell them so, rather than making other plans without telling them in advance. At the moment, they might feel you're letting them down at the last minute. (Though driving home from a wedding to see them seems INSANE.)
Can you break up the routine a bit? Can you gradually wean them onto two weekends in three?
Can you mention how now DD is a bit older you feel you can do more exciting things and you'd like to try to use your weekends in new and different ways?
I know it's hard if they work, but could you see them sometimes in the week instead, even without your DH? Fair enough, they're not people you'd fall over yourself to spend time with, but then you get a weekend (or two!) 'off' because they've had their grandchild fix. Could they meet you late for coffee and cake at 5? Could they come round for DD's 'teatime' and the mess fest?
Also, although it's tough now, their interest will stand you in good stead later. I found it was v hard at 7 months to leave a (still partly) breast-fed, demand-fed baby with anyone (don't know if this is the case with you, of course) but once DS was 1 or so, he was very happy to spend whole afternoons, or days, with his grandparents while DH and I did things together.
And later on, it will also be easier for your DH to take DC to see his parents without you - blissful time to yourself!
I guess the answer is, as always, keep talking to your DH, and to PIL, and good luck!