Even minimalist, I'd use advanced for a number of these, especially the ones that are animals but particularly mighty.
I also suggest aurochs insead of goat, for the bull. Advanced aurochs is CR 3, so doesn't affect your conclusion any, but definitely more appropriate stats.
I generally like the approach. To recall some stats from long ago, when I did Beowulf I wrote him to be matched against an almost bog-standard crag linnorm.
I will say that I think a fairly large challenge to lower levels would come from the war stories and heroes cutting their way through swathes of nameless opponents. Choking rivers with so many dead that they no longer reach the sea, and all. Admittedly though Scamander is not
the most enormous
Secondary, feats of strength are not strictly level-based, but do suggest very high ability scores. Stuff like:
But Turnus shakes his head: "Your burning words,
ferocious Trojan, do not frighten me;
it is the gods alone who terrify me,
and Jupiter, my enemy." He says
no more, but as he looks about he sees
a giant stone, an ancient giant stone
that lay at hand, by chance, upon the plain,
set there as boundary mark between the fields
to keep the farmers free from border quarrels.
And twice-six chosen men with bodies such
as earth produces now could scarcely lift
that stone upon their shoulders. But the hero,
anxious and running headlong, snatched the boulder;
reaching full height, he hurled it at the Trojan.
But Turnus does not know if it is he
himself who runs or goes or lifts or throws
that massive rock; his knees are weak; his blood
congeals with cold. The stone itself whirls through
the empty void but does not cross all of
the space between; it does not strike a blow.
Just as in dreams of night, when languid rest
has closed our eyes, we seem in vain to wish
to press on down a path, but as we strain,
we falter, weak; our tongues can say nothing
the body loses its familiar force,
no voice, no word, can follow: so whatever
courage he calls upon to find a way,
the cursed goddess keeps success from Turnus.
If we assume chosen men = Str 13 and 5e rules, we've got Turnus lifting a boulder weighing in the neighborhood of 2300 lbs. And not just lifting it, but tossing it around. And managing this (though not fully) while being level/str drained by a hostile demon.