Welcome!
You already know about felt being a red mite haven so that's great.
Creosote does kill red mite really well, if you happen to have a spare coop for a few days after treating the birds' usual coop each summer or so it'll be fine. I personally don't use creosote (I have plastic coops) but if I understand correctly it has to be dry before the birds can go back in - easy on a day like today.
Will the coop be movable? If not, will it be near water or even electrics should you want to use electric netting? Mysteriously, water is a lot heavier when it has to be carried than when it comes out of a tap...
If the run is covered and you add some sort of bedding like woodchips it should not suffer too much in wet weather (any grass in the run will be gone before you can say scrambled eggs). Dry bedding = much cleaner run.
How about foxes when out in the garden and rat proofing the run? Any birds of prey to worry about?
For 4 or 6 birds the run would be ok for part of the day (say early morning and after dinner and let them out free ranging the rest of the time - they'll happily gobble up the slugs in your vegetable garden but the seedlings too
Birds don't get cold but do get hot easily, so that hebe bush might be just the thing for them to hide under on days like today. Or add a dark roof or shade netting to the run, a big christmas tree with its root...
Having said that, if there's any clear perspex or similar laying around that can be fixed to the run, they'll appreciate a few windbreaks in winter.