Your time estimate is about right , without stops, if you plan that longer route via Lochgilphead to Glasgow.So much to see and enjoy, it will easily take you all day.
Is this a spring/ summer trip? If so, I’m a fan of the lovely Loch Melfort Hotel but it’s a bit too soon on your journey to stop.However, if you are love beautiful gardens, the adjacent Arduaine , run by the `National Trust , is glorious and in a gorgeous location.It is particularly renowned for its rhododendrons.
Carnasserie Castle is a ruin just before Kilmartin Glen.Haven't been in to the new Museum since it opened, great area for early history including Temple Wood and various standing stones and burial cairns.Also consider a short walk up to Dunadd, the crowning stone of the ancient Scottish kings.
Crarae Gardens are on Loch Fyne though mainly a woodland garden albeit with some nice rhododendrons and magnolias in season.There is a Neolithic burial cairn near the visitor centre.
Inveraray is an attractive place - the castle in a stunner set in beautiful grounds. Brambles Cafe has quite an extensive menu although another 10 mins or so south lie two choices for lunch or cake etc - the Loch Fyne Oyster Bar (has quite a wide menu - may have to book) and the cheaper, small Tree Shop Cafe for home made soups and good cakes.Nice gift shop too.
If time allows, the best way to see Loch Lomond is on one of the short boat trips from Tarbet though in peak season you will probably need to book ahead.
Firkin Point has parking and you can do a short walk right by the loch from there, as far as you want.Lovely views.
Luss is a pretty Conservation village in a lovely setting, with the Coachhouse Coffee Shop for light lunches and cakes (excellent home made meringues!) There is a short walking loop through the tiny village (always busy and paid parking) - the church is delightful.
Duck Bay Marina also offers food.It’s a fairly ‘bling’ interior but has an unbeatable outdoor terrace right on the loch.Fab on a sunny warm day. Great ice cream there too.Big menu and a coffee shop too.