Hey there
Mice can be tricky, my first suggestion would be not to leave the food in 24/7. Put it in in the morning and then take out before dark. Leave hay in for them all the time, but if you’re leaving out pellets or a muesli mix for them all the time that’s going to be the biggest draw for the mice. Otherwise you may need to put some traps outside of the rabbit area, whether they are “permanent” or humane traps where you can release them later some distance away elsewhere on your property. I would probably suggest the humane traps given that your chickens roam around outside the pen and you wouldn’t want one of them to get a foot caught.
Another option, although much more laborious and difficult would be to mouse proof the enclosure. You would need to dig mouse mesh into the ground around the border of the fencing and up the side a little, followed by a solid surface around the outside (that they can’t continue to climb up). It would be difficult to do though, especially around the door etc.