I can understand you feeling resentful that your DS and DIL have expected a lot of help from you since your first GC was born, especially as you live 350 miles away. The combination of travelling back and forth and running around after a toddler must be tiring, even for someone as fit and active as yourself. If your DS and DIL are frequently asking you to do this or simply assuming rather than you offering, then that's not ok and it does need to be addressed before your new GC arrives. Otherwise the demands on you will only increase, and so will the resentment you are currently feeling.
BUT, having said all that it would be spectacularly unfair to say nothing, quietly letting your resentment build over many months, then punish your DS and DIL by deliberately being out of the country for your GC's birth knowing full well that this means DIL will have to labour alone and DS will miss the birth of his child. Whilst that would certainly be making one hell of a clear statement, the phrase "using a sledgehammer to crack a nut' springs to mind.
In your shoes, I would plan to attend the festival next year and make sure I was around to look after my toddler GC when DIL goes into labour. But I would also instigate a private conversation with my DS, well before the new baby arrives. Explain that although you love being a GM and are very much looking forward to welcoming your new GC, you are not getting any younger and simply cannot continue to provide the same level of childcare and domestic support you have been. Tell him what you think is reasonable in terms of frequency of visits, but explain that you want and need the flexibility to travel, to go to events and to pursue hobbies while you are fit and well enough to do so. Tell him that on reflection you think his idea of hiring a Nanny is the right thing, that way DIL won't be so constantly overhelmed and when you visit you can really enjoy your GC rather than feeling like The Help. Tell him you want to think carefully about how this is communicated to DIL, because you value your relationship with her and you don't want her to feel ashamed for struggling. A simple, casual "I've been thinking, we should get a Nanny. We'll need help when the new baby comes and Mum's not getting any younger. I don't want her to wear herself out" might be kinder and less inflammatory than giving her a detailed account of your conversation.
You can set these boundaries kindly but firmly, without the need for big relationship-rocking gestures.