The routine gets done, the homework gets checked, and then the screen comes on. Not before.
As many warnings as you choose, at the intervals you choose. Then it ends. Without you being the one who says so.
Minutes don't roll over and can't be negotiated. The moment they can, you're back to bargaining, and ending the bargaining is the product.
End screen time from your phone, for one child or the whole house, immediately.