You could do a mini-tour of the Outer Hebrides in that sort of time.
I'd agree with that. We did a mix of road and offroad on a similar trip a few years back. We got the ferry to Lochboisdale. Rode up the west coast of South Uist - mainly on tracks through the machair and a fair bit on the beach itself. Had a bit of an explore of Loch Sgioport then on up through Benbecula and Grimsay to North Uist and got the ferry up to South Harris from Berneray. Rode the west coast route to Tarbert via Luskentyre and then back down the east coast on the Golden Road (stunning). We got the ferry back to Berneray and then caught a bus with our bikes down to Eriskay where we got the ferry over to Barra & Vatersay for a couple of days.
Whilst on Harris I attempted the first bit of the Rhenigdale loop on a loaded bike and am ashamed to say I had to abandon it. If you wanted to overnight it, it's be best planned as a standalone ride with light bikepacking kit I should think.
Awesome trip from start to finish - would love to go back and explore some more.