Edinburgh to Luss

The most direct route which is fairly scenic is via Stirling and the A811 to Drymen, Balloch then Luss.

Stirling is a must stop only if you are interested in the Castle and/or the Wallace Monument and potentially the Bannockburn Centre for history of the battle.The Castle itself, including getting parked, will taken around 2 hrs out of your day. Another 1 hour at least for the W.Monument (has its own parking).

You also have the opportunity to detour into the more scenic areas of the Trossachs part of the NP by heading on to Callander then along to Loch Katrine past Loch Venacher. If it's a nice day, that would make for a fine detour.There are boat trips on the loch (the Sir Walter Scott steamship too) - longer or short trips.You can also simply walk along the loch shore on the excellent minor road which is virtually traffic free (Water Board/residents only road apart from cyclists.)Beautiful area.
Before the turn off for Loch Katrine pier is the lovely little Trossachs Kirk, well worth a short stop.

Then head over the Duke's Pass (great views) to Aberfoyle and on to Drymen (can visit the village via a short detour) and Balloch and Luss.

Balloch Park is lovely and offers good loch views though you will get those aplenty from Luss!

This is incredibly helpful. Thank you!

This thread is quite timely. We are going to be driving from Edinburgh first to Inveraray and then a little bit further. I am debating whether to drive along the M8 to Glasgow and then up the west coast of Loch Lomond, passing by and/or stopping in Balloch, Luss, and Arrochar, or whether to drive northwest on the M9 to A84 and then A85, passing through Callander, but then going down A82 to Arrochar and on to Inveraray. Google Maps suggests it's a difference of only 30 minutes or so driving time, so I was trying to figure out which would be more scenic and perhaps have less traffic.

You would have to be very careful about when you headed along the M8 to Glasgow because during rush hour the tailbacks are really awful though they are not quite as bad if you branch off to the M74 which goes round the south-side of the city before rejoining the M8 at Govan.Avoid the Kingston Bridge route which cuts through the centre of the city, like the plague.
It is not a scenic journey at all until you approach Balloch and Loch Lomond.
You then have an easy drive on a good road up to Tarbet and on to Inveraray, scenic all the way.

The Callander/Crianlarich route does seem a long way though it is certainly scenic once you reach Callander. The issues with that route is, as you head down to Tarbet on Loch Lomond, you will have around 20 mins of narrow, twisting road used by coaches and heavy goods vehicles which can be hair-raising if you are not used to it!

Your other alternative is to head for Kippen and Drymen via the A811 and approach Balloch and Loch Lomond's southern end allowing you to enjoy some lovely stops by the loch - Balloch Country Park is nice for a stroll, good loch views; Luss village is pretty and in a gorgeous location though gets very busy but - no wonder. Duck Bay Marina is a huge operation but offers a fab terrace right on the loch.
Firkin Point has parking and allows a stroll by the shore too.
A boat trip from Tarbet would get my vote.