Not read all the posts, but I did mention crossing our wild rabbits with a big meat breed buck and he about had a fit. I was joking, I think.
I used to walk through some fields on the edge of Stirling. They had clearly had pet rabbit genetics mixed in. I know you always get the odd black one in the wild but these were all different colours and shapes and pattens mixed in with wild types. There were all the typical natural predators but bunnies seemed to do alright for a couple of years. Then mixi flared up and just wild type present in more recent years.
I don’t think there’s much point in changing the genetics, just do what you can to conserve the wildlife you like/want. Some people like song birds and go to lengths to feed them, plant fruit bushes, hedges and put up nest boxes, shoot crows and magpies, etc. If you want more bunnies have a read or watch your land. Work out what you have (camera trap, trap, track,) find out what the constraints are and consider making scarecrows to put off foxes, maybe provide winter hay, etc. Maybe think about growing crops like brassicas and only give them access in the winter when they’d otherwise be short of food, etc. They say to hit them hardest in winter, out of breeding season to control numbers, so to get more you probably want to provide them safety and food in the cold months. If you build it they will likely multiply! (Until mixi or one of the other unspellable diseases kills the lot!). You said you dont have any neighbours who’ll mind which is good, this would be completely unacceptable for the majority of country folk!