If you want sea and islands, it needs to be west coast, so midges are a fact of life, but they will only bother you when the wind is less than 4 KPH.
Strongly suggest you don’t go to an island if it’s your first time in Scotland as you will want to explore different areas – if you’re on an island, however stunning, travel is very restricted.
Oban area is nice, with good ferry access to the islands but is not quite proper highlands. Somewhere around Fort William might be best – lots of places like Glenfinnan on the road to Malaig. Fantastic walking on your doorstep, Ben Nevis and the Mamores, Glen Coe, Ardnamurchan, ferry to Mull ets. True highland experience.
We have rented cottages probably 30 times in Scotland – still do it even though we now live up here. Some of the accomodation is a bit variable, so if you want good modern standard do not go for anything less than 4* STB rating