Did the Leadership of the Yahrushalam Assembly Change from Circumcised Hebrew Leaders to a Gentile Leader After Hadrian?
Witness: Eusebius, Ecclesiastical History 4.5–6 · Period: before and after the Bar Kokhba revolt
What the ancient record says
Eusebius (4.5): the assembly at Yahrushalam consisted of believing Hebrews from the emissaries' period until the siege under Hadrian; he lists a succession of leaders belonging to "the circumcision." (4.6): after the devastation and Roman transformation, the assembly was composed of Gentiles, and he names Marcus as the first leader after those of the circumcision. Before: believing Hebrews, leaders of the circumcision.
After: a Gentile assembly, Marcus its first Gentile leader, Eusebius himself marks the transition. No surviving Roman document says "we are transferring control from Yashar'al to Gentiles", that wording exceeds the evidence.
What we can establish is the sequence: Rome crushed the revolt; Yahudim excluded from the city; the city became Aelia; earlier leaders of the circumcision; postwar assembly composed of Gentiles; Marcus first Gentile leader. Limitation: Eusebius admits he had not found the chronology of the earlier ~fifteen leaders preserved, he claims the succession tradition, not the complete chronology.
Read it for yourself. Eusebius, Ecclesiastical History 4.5 then 4.6 (back to back, the contrast is the evidence).
Evidence strength: Very strong for what Eusebius reports; limitation: incomplete chronology of the earlier succession.