Moving from 'once' or 'has' to another 'has'
Dear Grammar people,
I was wondering if you can move from a sentence such as 'they were once .... then they had....
For example, if we take a look at the beginning of a poem I wrote earlier:
They were once slime-morsels, contained
in a thick black soup for frogs and newts,
but they had clambered out of the deep sopping murk
where most stayed swallowed and churned in darkness,
and on stems their minerals morphed into azure stones,
with sockets for veined-glass-wings: set for another world,
set to pump up, to flap above the algae of the clutching earth.
Is the progression okay?