CLAIM:
“Those guys in Acts keeping the Sabbath were already Jewish, we’re not”
The Sabbath is/was practiced and was originally intended for gentiles also:
Isaiah 56:6–7
“Also the sons of the foreigner who join themselves to the LORD, to serve Him,
and to love the name of the LORD, to be His servants—
Everyone who keeps from defiling the Sabbath, and holds fast My covenant—
Everyone.”
(Emphasis added to show that the promise includes everyone who joins themselves to the LORD and keeps His covenant.)