I think it's important to differentiate the difference between wild camping and informal roadside camping, the latter is pulling up by a layby or car park and pitching up near your car, so you can carry more luxuries such as gazebos, alcohol, BBQs etc and tends to be associated with dirty/fly camping due to a minority of idiots. Whereas wild camping is essentially backpacking, carrying all your gear and pitching somewhere you can only reach on foot.
The Cairngorms are a great place to go hiking and camping, Linn of Dee is a personal favourite of mine as there's lots of options.
As for the midges, they'll be slowly dying off from now, and should be hardly any by October.