I’ve got two sets of slx brakes, the 675 ones.

One set has a lovely feel with lots of modulation so you can feather the brake easily. I’ve had these from new and got on really well with them.

Tthe other set I just bought second hand to replace some shoddy brakes with something similar but this set feels very on/off requiring very little lever movement to fully engage.

What do I need to do to make the second set feel like the first?

Possibly let some fluid out of the second set. I like my brakes to feel very on/off and often over-fill them to achieve this.

I thought might be the go but wasn’t sure!

It’s possible that the pistons have advanced too far

If you can, use some old pads and a screwdriver to lever the pistons right back into the caliper then refit the pads & caliper and visually align it over the rotor. The first few pulls of the lever will be very mushy but as the pistons advance the lever will firm up