You cannot scale up pixel-based raster images without quality loss. That has nothing to do with Photoshop, it's just the nature of pixel images. If you need big pixel files, you need the hardware to handle it.
But maybe the file doesn't need to be as big as you think. It's a common misconception that everything for print has to be 300 ppi at any print size. It's one of those persistent myths that never go away. But 300 ppi only applies to books and magazines to be seen at arm's length. Bigger prints will be seen from farther away, so the ppi requirement drops accordingly. A large wall-sized banner or roadside billboard can be 10 ppi.
A1 is 59 x 84 cm. At 300 ppi, that's 7024 x 9933 pixels. Yes, that's a big file, but not so big that a decent current computer should struggle with that. If this brings your machine to its knees, you should think about some hardware upgrades.