That is annoying. But as I mentioned you will need all the evidence you can collect in the supporting such as these links. Particularly stating the local council calling out the walking exercise part of the park.
My suggestion
""So when you nominate a possible waypoint.
This bit is only if using phone to nominate. But if using PC and on the wayfarer website Niantic Wayfarer then feel free to ignore 
but the principles still apply
Write brief title, description, supporting (make sure this text is not rubbish but at least relavent). Select what it is (cafe etc) - with photo. /you can take photos before hand. In particular for main photo. Edit so they fit best. Ensuring main part of image showing of the nomination is kinda 4x4 (square is best) Centre what the POI is. you can then upload from your gallery when creating the nom.
BUT DO NOT Upload now. Use the Upload Later. Get home. Log into WayFarer on your PC/Laptop. Go to the Contributions Section. Now on phone click upload now. Refresh the contributions every now and then and after a while your nomination will appear. Put it on Hold. Once in hold you can edit at will.. ((( note every now and then Niantic will grab your nomination before you edit but hey that happens - not often - I just roll my eyes - take it and nominate again 
)))
Once on hold. This means you can rewrite the title, description and supporting information. You can do your research. Include weblinks etc to your nominations (include the link to the Niantic criteria clarification for parks) - I even make it easy for the reviewer with a direct link to google street view). Remember in supporting.
Select one of the criteria. Say It is a great place to explore. Because of X Y Z. Or Great Place to Socialise. Because. and remember evidence to support being explore, socialise or exercise goes a long way. DO NOT say make a great pokestop. Or we need more stops Or great for the game etc. That is not evidence or supporting information and is considered trying to sway the reviewer
Before you release hold to put your nomination back into voting. Use the nomination support forum. Include all text, images and include the address. And ask hey how can I improve my nomination. I have often used this forum to great impact. Steadily increasing the quality and subsequent success of my nominations
Once you have your feedback and you ready click - release hold.
Yes boring to have to repeat a nomination. But I have found well researched, supported nominations go a long way 
with far great effect and for unsigned parks work very well. Good luck @idionz
@Jesperkaa86 same advice on your park nominations. If you don’t already do it