Hi and welcome from me too.
In a lot of respects old brick or stone sheds are better as they are more predator proof. My hens use an old stone stable. I find rat traps baited with peanut butter are very good but you have to make sure to put them somewhere that the chickens and any other animals can't get at them. Rats can't resist peanut butter!
I use straw on my block paved concrete floor. The chickens love to rummage through it, but I haven't tried sand so I can't comment on whether that would be better. I also use straw in the nest boxes. I've tried shavings but they tend to scratch it out all over the place and if you have a sand floor I would imagine it will all get mixed in if they do that. I also sprinkle a bit of Diatomaceous Earth (DE) down on the floor before I put straw down and use a soft brush (a dustpan brush is good) to dust into all the nooks and crannies where mites hide... but wear a mask or scarf over your face to prevent breathing it, if you use DE. I change the straw out and clean up once a month or so. Oh and drop boards under the roosts are a major benefit if you don't have them already.
Can you post photos of your girls so that we can see what breeds you have? Like you, my chickens get to free range every day. I've had a couple of fox attacks but thankfully not been too major and the chickens would take so badly to being penned in when they are used to their freedom, that I don't have the heart ot time to build them an enclosure.
It is wonderful getting to know all their little characters and finding out what their favourite treats are. It's funny how different their tastes are. Mine live apples. If I cut an apple in half and hold it out for them, they will demolish it within a minute. You have to hold tight as they can be pretty enthusiastic. My neighbours hens, on the other hand, wouldn't touch an apple even though they are penned up.
Anyway, I must stop rambling on and get on with some chores.