Can you use "Isle" like "Island" when referring to a non-specific Island?

You have the right intuition about the word 'isle':

• technically, it refers to the same concept as an island. So it is technically correct.
• at the same time, it has particular associations, that 'island' does not, with being poetic or flowery or in fixed phrases or very particular islands ('Isle of Wight').

So you would be totally understood if you used it instead of island, it is technically correct, but it would sound slightly fancy, like you're trying to be poetic. The point is that the different word will invoke a slightly different feeling in the hearer.

I don't think it is standard usage, according to the Oxford Learner's Dictionary isle is used mainly in specific contexts like:

used especially in poetry and names to mean ‘island’

the Isle of Skye

the British Isles