It seems to me that, with four people splitting the cost, you might find a private tour to be no more expensive than a quality group tour. Rabbies does get excellent reviews, but if you worked with a travel agent (probably in Scotland), you'd be able to structure an itinerary that would align more precisely with the desires of your particular group.
I would encourage you to allow more time if you can; there's a lot to see in Scotland, and it would be nice not to be so rushed. There's the issue of weather--specifically, rain--that can affect plans for rural sightseeing by less-than-hardcore walkers.
If half of the group is limited to 2 weeks but the other two are in a position to add on more time, I'd recommend considering that. I did that with a friend in 2019. She could only manage 19 days, but I had 26. There was a lot I wanted to do in and around Edinburgh and Glasgow, so I added extra days in those cities before she arrived.