Since my last mortal sin, i've been worried about having the secret 666 marked on the forehead and hand. IT seems to come around the time where there will be global buying and selling problems without wearing a mask or chip or whatever.

I'm not sure if a good confession can rid the mark, but its in the old testament as God's wrath in end times over its people a silent chastisement.

It is not clear to me that those who consent to having the mark imprinted on them would for that reason be considered to have apostatized.

I mean, they will certainly have enslaved themselves to his will, but do they thereby leave the faith and enter into the service of the devil?

In an indirect way, maybe, because those who don’t resist (just like with the mask) allow and facilitate, however unwillingly, the plan of antichrist.

But if there is no disavowal of Christ and His one true Church, would a confession even be necessary or appropriate?

It seems you would be doing something stupid and/or immoral by getting the chip or vaccine (if abortive) but that aside, it is less clear to me there is anything else against the faith?

This will be world enslavement at the natural level.  The final tribulation and persecution of the faith and faithful will come later (and the chip will probably have a part to play in that, so maybe there is an issue there?), but it is not all clear to me.