For the end of the older synapsid stage, I’d use late caseids as your marker. Caseids are basal/non-therapsid synapsids that persisted into the Middle Permian, with the last known forms around the Roadian–Wordian range, before disappearing as therapsids took over.
Last Non-Therapsid Synapsids
~262 Million years ago (+/- 2 million)
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