Are you thinking you need to start/end at Railway Stations? That sort of sets some bounds. Happy to Hostel/ Bunkhouse/ camp/ B&B?? e.g. there’s a bunkhouse at Inverie in Knoydart that’s just a short walk from the pub.
Or look further North towards Achnashellach on the Kyle line. You could easily spend four days off road in that area. There’s a private hostel (Gerrys) at Craig, plus bunkhouse and hostels at Torridon, Kinlochewe and Achnasheen.
Devs Cairngorm option has lot of possibilities too.
Geoffs idea of the Western Isles tour shouldn’t be dismissed. Set up a rigid MTB with some slicks and take to the roads of the Outer Hebrides. Problem for you would be fitting it in to the time available as it’ll take you a day just to get to Barra.