I can’t decide.

I think not. If you isolate someone, you cut them off from other people. If they cut themselves off, they are self-isolating. In fact, if someone has been told to keep away from others by Test and Trace, they are not, strictly speaking, self-isolating.

Regardless of whether you isolate voluntarily or are instructed to do so, it’s still isolation so is the ‘self’ not superfluous?

But "isolate" is usuallly transitive; you could isolate your pet or a sick plant in your greenhouse. Just saying "I am isolating" isn't entirely clear, whereas "I am self isolating" removes any doubt.

You can isolate a live wire or cable or you can isolate yourself = “self isolate”.
You could isolate a patient with an infectious disease (we used to have “isolation hospitals” ) but the verb “isolate” still has an object ie who or what is being isolated
So self- isolate is not a tautology, if you miss off the “self” it is left open “who” or what you are isolating

For the avoidance of doubt one could say ‘I am in isolation’ removing the requirement for ‘self’. Technically we weren’t self isolating though as it’s been compulsory under certain conditions Does ‘self isolate’ not indicate a degree of personal choice?

Surely in the expression self-isolate "self" is the object of the verb, which as others have pointed out is transitive, so should always have an object.

"I am self-isolating" had won over "I am isolating myself" which is what I take it to mean.

In this sense we are still self-isolating even when we are following restrictions imposed on us by others.

Tautology is saying the same thing twice, in different ways.
So, 'self isolating' is a tautology. The word 'self' isn't needed. 'I am isolating' is enough.
(Admittedly, from someone who has just this minute looked up the meaning of 'tautology.')

Does ‘self isolate’ not indicate a degree of personal choice
It seems so. It's implicit that one has chosen to do it. The alternative is forced isolation, or imprisonment.