I do not agree with your first paragraph. When God established the Sabbath and sanctified it, was at creation. I believe the Sabbath was established and ordered with Adam even though scripture does not say it. Why would there not be verbal instructions to pass on? Many other things were established way before the law and written history.
Your second paragraph about the proper way to observe Shabbat: you could write a whole book about that. Basically keeping the Sabbath is to not seek your own pleasure, but to love God and others as yourself and take time to show it.
See I thought about that a ton. Just like one should not murder, the covenant with Noach was advanced and “built upon” by the Torah so now there are many more laws but those were obligated to Jews as recognized by your second paragraph.
But if you mean that Christians should get off the Sunday church days and back to Saturdays, sure but still they are not obligated but that doesn’t mean it is not profitable! Like Paul says, if you are WEAK in faith, forget about the days and festivals.... focus on what you struggle with (i.e. maybe a drug addiction, maybe just trying to go a day without hating people etc). BUT for Christians who want truth and a greater connection to H-Shem.... welllll come here and I’ll show you Shabbat!!!!
I LOVE SHABBAT!!!!!!!!! It’s not only a “Holy-Day” every week BUT WOW.... POWERFUL!!! Singing psalms all day, using all 6 days to prepare a feast for G-d’s Holy day.... the people.... ahhhhhh!!! Lol! I want to share it with everyone....but not everyone is ready to see its beauty.