Astikāya is merely a formal variation of the same word we know as sakkāya. So it seems clear it was a term the Buddha drew from the Jains, or from the ascetic teachers more generally.

Astikāya means “existent substance” or “ontological category”.

We can draw upon this, and keep a broad consistency with the handling of astikāya in Jainism, by rendering sakkāya as “substance” or “substantial reality”, and sakkāyadiṭṭhi as “substantialist view”