I'm from the UK. I recently went on holiday to Copenhagen and didn't have to use cash at all because I could use my debit card wherever I went. Will it be the same in Sardinia? Do all restaurants and vendors take card payments? Are there ATMs in all towns?
All towns will have an ATM, though it may not be accessible 24/7. I would expect most restaurants and shops to accept cards, though small ones may not.
It would be reasonable to expect to mostly use a card, but don't rely on it.
Don't expect small villages shops to accept cards. Some of them yes, but especially for low amounts, shops' owners will not accept cards.
ATMs are everywhere, and almost everywhere accessible 24/7.
In terms of card acceptance in Europe I’d rank a few countries basically like so from most able to live just on a card to least:
Tier 1: Sweden Tier 2: UK, other Nordics Tier 3: Germany, France Tier 4: Switzerland, Italy, Spain
Corsica is France though in many ways more akin to Italy. In France to a large extent and particularly in Italy you’ll often find places that don’t accept cards in the countryside. In the large cities however you should have few problems.
Overall however it is generally considered these days that the best way to convert money when travelling is to simply withdraw from a foreign ATM using your card- particularly good if you can get an account designed with this in mind.
So I’d say to withdraw a decent amount of money from a major bank when you get there but you shouldn’t need to use it too much.