A point about 'eternalism'. According to Bhikkhu Bodhi's commentary on the Brahmajala Sutta, 'eternalism' is best understood in the context of the religious and cultural world in which the Buddha taught.
Bear in mind that the Buddha himself is said to have recalled all of his previous existences on the evening of the enlightenment. There were other 'ascetics and sages', as they are called in the texts, who likewise remembered or claimed to remember previous lives.
Now, a characteristic criticism of eternalism is of given in the Alagaddūpama Sutta, regarding those who claim that ‘after death I shall be permanent, everlasting, not subject to change; I shall endure as long as eternity’ - this too he regards thus: ‘This is mine, this is my self.'
Elsewhere in the Brahmajala Sutta, such views are described like this: that 'The self and the world are eternal, barren, steadfast as a mountain peak, set firmly as a post. And though these beings rush around, circulate, pass away and re-arise, but this remains eternally. Why so? I have by means of effort, exertion, attained to such a degree of mental concentration that I have thereby recalled various past existences. ...That is how I know the self and world are eternal...’.(DN1.1.32)
In the context of a culture with an acceptance of the reality of rebirth, there might easily be a tendency to regard the aim of the religious life to reach such a state of permanence or imperturbability, or to secure an endless succession of rebirths, through the appropriate disciplines and sacrifices. This is what is criticized as 'eternalism'.
(The Brahmajala Sutta can be found online here
. It is the first (and longest) book of the Digha Nikaya and by no means an easy study).
So, in these discussions about 'nihilism' and 'eternalism', it's important to see what is being rejected as 'eternalism'. I don't think eternalism is a rejection of the fact of agency or of intentional action; it's the rejection of an eternal and unchanging self that persists for aeons of lifetimes.
Nihilism, on the other extreme (which as Bhikkhu Bodhi comments, is far more common in today's secular-scientific culture and which many people feel is validated by science itself) goes to the opposite extreme of equating the self with the physical body, meaning that at death, there are no further consequences of actions (or karma). There were materialists in Buddha's day also, they are among those whom the criticism of nihilism is directed at.
These are what I understand as the 'extremes' that the middle way is avoiding.