I think you stand a much better chance under island-wide voting than you ever would have done in district elections.
At district level, if you just appealed to people about one thing, you would never have gotten the amount of votes you’d need to be elected. At island level – depending on the issue you focus on – you just might. (It would still be a gamble, though.)
But I would never recommend basing your campaign on one issue alone. I still think you’ll need to convince people on a number of fronts – not just to prove that you’re interested in the same things as them, but also to demonstrate that you’ll be a competent member of government, capable of participating in a diverse range of debates.
(The other side of that is, if there really is only one thing you’re interested in, is politics the right job for you? By definition, the majority of your time is going to be taken up with things that aren’t that issue, as you’re engaged across the whole range of government business. It’s okay to have priorities for your term – I certainly did – but you also need to be prepared to get stuck in to whatever comes your way.)
Don’t be afraid to tell voters about your priorities. I think your manifesto can tell your story and explain your priorities much more directly than perhaps was possible in previous elections. But you do need to give voters enough substance, on a broad enough range of things, that they know you will be a good States Member all round, as well as a passionate champion of the things you care for most.