So, the problem. WE - my DH, DC (now in their teens & not particularly close to my parents) and I have become the focus of their time
Absolutely not. & this won't happen unless you choose to allow it to. You can be busy and have plans etc, or simply not want company.
This is a prime example of parents who don't really make time for their DCs, or are very 'hands off' citing 'not getting over involved/I let them live their lives' as an excuse. Then when said parents get old, you're supposed to do your duty.
I wouldn't.
My parents split up and my dad moved abroad when he was 55. He has a partner, nice life together. But fast forward, he's 80 now and maudlin about missing his adult children, and their children. He goes on and on about it, saying he has to watch other people there with their children/grandchildren around them. He and partner can't really get around much any more, especially as he's no longer well enough to drive.
We visit every few years. But he chose to move 1000s of miles away and didn't think of us missing him and his grandchildren not having a chance to get to know him well when he upped and left. It's like they have a lightbulb moment tinged with selfishness, when the elderly years arrive.
So, sorry - not my problem. I don't play the 'duty game'. & I feel you need to adopt same attitude to your parents. There's no need to be horrible to them, just don't make your life about accommodating them in a way they never did for you.
Mum remained in UK, I remember her airily saying she wasn't the type to 'sit around babysitting grandchildren' (as if Id asked her to!), when I was pregnant with DC1. As the years have passed she spoils them rotten, and seems unable to spend more than a day or so without speaking to them on the phone. They love their gran, she treats them well and I'm glad. I still remember what she said though, and on occasions I asked her to babysit she made me feel like crap. When I arranged my own regular babysitter (as I worked odd hours), she was enraged.
I clearly remember my parents when they were young, vibrant and active. We all get old, that's what people forget.