Stay wet palette is your best bet. I make my own by using a tray with a bit of a lip, put a few sheets of wet paper towel in and a layer of greaseproof paper on top, mix the paints on top of this. Keep the paper towels wet. Cover it over with cling film when you're taking a break, making sure its airtight. I've had paint last a week or more without drying out by doing this.

They make a drying retardent but acrylic drues fast and flat which can be a good thing so it won’t look built up cause it’s flat.

Use retarder or another medium