I agree with Joolio and everyone else who has commented.

I recently did the trip up to Lewis and yes if you're lucky with the ferry times you can pop over and see Callanish, and then back the same or next day. But you'd be mad to. There are dozens of sites on Lewis and then there's all the beaches (which I loved). Take your time.

The main roads up to Ullapool is motorway up to Stirling then very good A road.
Across land from Ullapool to JO'G would be a fecking nightmare whichever way you did it - over by the east coast or along the west's tiny roads - nah not my idea of fun!!!!

Timings are Extremely dependant on ferry times. At the time you're going it might be OK, but when I went only a few weeks ago they were still on winter timetables over to Lewis ie once a day.

Never been to Orkney so couldn't chose between there or Lweis. But I bet there's loads to do on both.

Some bloody holiday if all you'll be doing is ticking a list!!!
My advise do one but not both. Bloody hell, smell the flowers...

PS I presume it was a typo in your itinery when you said Callanish instead of Castlerigg on day one ;)