Shimano brakes normally feel rock solid to me, I reverse bleed with a syringe and a bit of tube.
Take the reservoir cover off, attach syringw to caliper bleed nipple and suck all the fluid out, then with fresh fluid refill thru the bleed nipple once the resevoir is full lock of the bleed nipple.
Replace cover and try, if they still feel a bit spongy hang the bike up by the front wheel and pull the levers back and lock off with zip ties and leave overnight.
That normally results in a rock solid feel for me.