The above is all accurate, though I'd add that there are also 'Sleeping Pods' on the ferries.
As
@oldwheels has mentioned, book early ! I go up every year to visit family and I always book as soon as the bookings open.
I've cycled quite a few times in Shetland; the roads are generally very quiet and it's worth mentioning, that you'd find that the road surfaces are in far better condition than just about anywhere else in the UK.
Look out for 'Cake Fridges', from which you can buy cakes, bakes, eggs, drinks etc - they exist in the most unlikely places. Last year, I was up in North Roe, which is almost as far north as you can go in Mainland Shetland; we did a very long walk and I was mightily pleased to find a fridge selling drinks (as well as cakes). Every fridge has an 'Honesty Box' for payments and the system does not seem to be abused, despite there often being numerous £10 & £20 notes.
Depending on your interest in Wildlife, it might be worth joining the 'Shetland Orca Sightings' group on Facebook. They make postings of Orca sightings and it's usually possibly to predict where they will be - either on the West or East of the Shetland mainland and heading North or South - the group members usually state the direction in which the Orcas are headed. They tend to stick very close to the shore, as they like to pick off unwary seals and they can be seen in the most unlikely places, including Lerwick harbour.
Shetland is ace ! Go there and you'll be sure to have a great time !