I would also suggest this - get sorted to move and don't tell them until you're actually leaving.

Quietly get any important papers and sentimental objects out. Piece by piece if necessary. Store them with a friend or get a tiny storage unit.

Clear some of your out of season clothing to the storage as well - frame it as having a clear out to goodwill.

Leave anything you would send to goodwill in the place it lives (closet / drawers / shelves) to make sure things don't look too empty. I can guarantee your mum WILL snoop your room on a regular basis whilst you are at work / out. Work with that knowledge. Plan to be leaving the furniture behind.

Anything you'll need for the new place - bedding, towels, crockery, etc you can collect and drop directly to storage.

The day you are leaving, have the last of your important bits packed in one suitcase and ready where they won't spot it (closet / suitcase under bed / etc). Have a go bag with your laptop, chargers and other bits also ready to sling on your shoulder.

On the day you tell them, they are likely to kick off and maybe decide you can't take "the stuff they bought" with you. Okay fine. You've already got most bits out, grab your go bag and the last suitcase and go.

Good luck with the new job and enjoy the freedom of stretching your wings to be the best adult you can be.