Taking a day off to rest is good, just as fasting is good; they both have benefits. Taking a day off to honor Him and to rest is not the same thing, just as fasting to diet is not the same as fasting as a sacrifice to Him, and including prayer.
One can debate what day God set aside as the sabbath, but it seems clear that there is one. If one sincerely believes it is Sunday then do so on Sunday. If Saturday then do so on Saturday. We are told to not forsake the assembly of fellow Christians. How does that happen if any day may be chosen? I’m reasonably sure it isn’t “remember the day you select as the sabbath, to keep it holy”.
I am amused by the irony that one of these is legalistic: what we must do vs. what we must not do.
Notice there is nothing in the law about what to do on the Sabbath. Only this: do nothing. But one is allowed to do good on the Sabbath, if necessary.