A day trip to Skye or Inverness is not feasible from the Central Belt (Glasgow, Edinburgh, Stirling). It takes me at least 6 hours to get to Edinburgh or Glasgow from my house in Skye and at least 5 hours to Stirling.
If you have 18 days you'd be better off planning at least a week in a Highland base, so that you can explore the Highlands (e.g. Inverness or Fort William). Even from Inverness it's 3 hours to Skye, so you'd maybe want 2 or 3 nights in Skye (if accommodation is available at this late stage assuming you mean August 2025).
You could then spend the remaining week in the Central Belt. I'd suggest Edinburgh (although it will be incredibly busy due to the festival), or failing that Stirling, from where you could do day trips to the other two cities.