You are correct that it is an assumption that the mind is no more than a material function of brain activity.
That the mind holds a viewpoint on four dimensional space and time implies that it is separate from it. It doesn't make much sense for the observer and the observed to be the same thing, does it?
It is, of course, a quite remarkable phenomenon that that brain is able to conceptualise it's own existence in space and time. However, just because this feels like a very odd state of affairs and is difficult to explain is no-good reason to drift of into metaphysical wish-fulfilling fantasies. Surely, we start from the simplest explanation based on the best evidence we have, do we not?
The best evidence we have is that all human cognition originates and ends in the physical form of the human nervous system. Therefore , whatever
we conceptualise the mind to be, that conceptualisation must
begin from the assumption that the mind is no more than a physical function of that nervous system.
Anything else, in the complete absence of evidence , is just making stuff up.