maybe this is an option, it came to me when reading this forum. (I still think affinity should make drawing to scale an option)
You make two drawings, 1: one to one, huge paper size, doesn't matter affinity can handle it. and a second one, 2: the size you want to print it. Where you place the first one.
Then you can scale the placed image to the right scale. (use devision in the size field)
When you change the first one, the second one will be automatically updated once set to linked in the resource manger. (you can keep both drawings open)
You can also double click the placed image and edit it. (but then it's not the same as the first, huge, illustration anymore.)