It would probably be wise to have two plans - one for wet with low cloud, meaning little scenery to see (it happens, and not infrequently) and one for fine weather.
I dislike the motorway between Edinburgh and Glasgow, even when it is working well. It is probably one of the most boring stretches of road in the whole of Scotland. And forget averaging 60 mph on it - there are long stretches of 50 mph through Glasgow and if the roadworks are still in place long stretches of 50 mph with average speed cameras through them as well. If you want to get to Edinburgh relatively quickly it is a sensible route, but if you want to see more of Scotland your idea of a country route makes more sense.
Based on your itinerary, in fine weather one option would be to miss out Glasgow. Instead you could turn west out of Glasgow Airport, cross the Erskine Bridge (if you glance to your left as you cross there are nice views to some of the Argyll hills), and then head to Stirling via Balloch. I have never been very impressed with the place or, indeed, with Loch Lomond, but some people seem to like it. I would instead detour to Aberfoyle and along Loch Ard (it is a dead-end road, so you have to head back the same route). You can head from there to Stirling (has an interesting Castle if you want a stop, and the Wallace Monument is not far out. Parking at the Castle can be difficult, particularly around holidays). Stirling to Edinburgh is straightforward on the M9, passing the Kelpies, which are visible from the motorway. You would have to exit the motorway at Grangemouth if you want to photograph them.
Depending when in May you are in Scotland, sunset in Edinburgh is some time between about 9 pm and 9.45 pm, and sunrise is sometime between about 5.15 am and 4.30 am, so you have a lot of daylight if you are willing to minimise sleep.