Recently swapped over my disc brake pads, but discovered that the pistons lightly scrape the rotor when not braking, and when I am braking it doesn't make much of a difference and it allows me to pull my lever all the way to the handlebar. Shimano Deore brakes btw
If you can pull the lever all the way to the handlebar while not getting much braking force, then it is very likely that you somehow got air into the system somewhere, and you need to bleed the entire brake system.
Perhaps when you were changing the pads, you pulled the brake lever while the pads were removed, which would allow the pistons to come out too far ? That would let air into the caliper, which would lead to the symptom you described.