I have two days to get from Portree to St Andrews. I would love to drive the lower route to Inverness and then on to Grantown-on-Spey for an overnight. Next day drive to the coast and down to St Andrews. My concern is how strenuous is the drive I have planned. My alternate route is to go pretty straight through Pitlochry and cut the driving time down considerably. This alternate misses, of course, all the lovely scenery along Loch Ness and the Spey River, and the coast south of Dundee. But I also know that I don't want to arrive in St Andrews totally exhausted so that I cannot enjoy that town and my golf game the following day. Thoughts from everyone are welcome. And I have thoroughly enjoyed and learned from the responses to my and others' queries. Thank you all.
Your first route suggestion is perfectly doable, although in my opinion the northern route to Inverness (via Achnasheen and entering Inverness via the Kessock Bridge) is more scenic than the lower route via Loch Ness. Loch Ness itself is largely shielded from very busy A82 road, and apart from a potential stop at Urquhart Castle is pretty uninteresting. The northern route has wilder scenery and much lower traffic levels and you could stop for a walk at Rogie Falls. Coming into Inverness via the bridge would enable you to bypass the traffic gridlock of the A82 approach and get straight out on to the road towards Grantown on Spey. Indeed when I plugged Portree to St Andrews via Grantown into google, it suggested the northern route to Inverness! The next day you could head to the coast via Banchory and then drive south to St Andrews. I don't think this would be any more strenuous than the alternative route below.
The alternative would be to head south to Spean Bridge where you'd turn left heading across country, before picking up the A9 to head south. At Perth you'd have the choice of heading across to Dundee and then south to St Andrews, or going south then east via Cupar to St Andrews. Personally with 2 days to make the journey I'd probably go via Grantown on Spey.
For my sins I find the route by Loch Ness very meh and the A82 is no fun to drive on.
You will be fine ,plenty of time for your trip.
Again the A9 from Speyside south has little charm compared to what you will see from Skye onwards .
The other option would be a make to Spean Bridge then Newtonmore missing out on Inverness .
But the minor roads through Speyside are very pleasant and often in the forest.
Nice cafe at Laggan and Feshie Bridge too