It sods to me like you really want the northwest of Scotland. There are several ways you could go. And indeed, you might want to divide it in two. Certainly there is the area near Skye. The Island is wonderful, you could base yourself in Plockton and from there can get to the Applecross area, which i've only seen in rain, but is wonderful and you can go further south to Skye or north toward Achnasheen. The Torridon area is beautiful and has lots of walks. You could get up as far as Loch Maree and Gairloch. There is an amazing garden just north of Gairloch.
Another area to consider would be Ullapool. There wonderful hikes in the Ullapool Hills and Inverpoly is really wonderful. Stac Poly is amazing and Suilvan is a mounting that just stands. You could of course, go South to Gairloch from Ullapool. We did a drive around the top of Scotland and spent the night in Lochinver. The drive we did the next morning out toward the Point of Stoer was unbelievable. Rock, rugged, the rocks almost inserted themselves into the road. I have not explored the area along the A837 toward Lairg, but it looks promising.
Further north yet, is Durness and then across the top in Tongue, Betty Hill and then Thurso. There wasn't much in Tongue, but oh my, the scenery! We did not catch the trip to the Cape of Wrath, which I deeply regret, but the cliffs and the mounts are stark and wonderful. The sands are amazing. This is Sutherland, and my family has ties to this land. The lochs are where the convoys assembled for the North Atlantic crossing. There is a wonderful book, The Sutherland Trail by Cameron McNeish which is about their journey through Northwest Scotland. I recommend it. Tongue is a nice little village. It has a great little hotel. Betty Hill is not far and that drive is wonderful. Betty Hill has a museum for the Clearances. And a very good thing to check out is Strathnaver. It had villages that were cleared and you can walk around in the area and see where the houses were etc. Thurso, is the ferry port and by the time you are there you are in Caithness.
You could alternatively base yourself in the Strathpeffer area. We used to do this all the time for my walking trips and from there you can do day trips to most of these places except the ones to the far north. And from there you can go east to Dornoch which is a different landscape, but beautiful. And, you can easily get to Glen Affric from there!
I had to open up my Tourist Highland Map to make sure I thought of everything. I wish I could have you here at my dining room table looking at it. It's covered with highlighted lines to where we took many of our walking trips.
PM or just ask more questions if you want more detail. I really liked the Foot Print Guide forf Scotland. I am sure that's where I read about the road near Lochinver.
One thing to remember, it all takes a bit longer than you think, as the roads are very wee, and there are such great views that you keep stopping.