Devtools device emulation does not reproduce slow CPUs, OS keyboards, or 3G networks. Test on a 2018 mid-range Android occasionally — the experience differs sharply from a laptop on fibre.
Test with one hand. The thumb does not reach the top of a 6-inch screen. Important actions belong in the bottom half.
Test with the device tilted. Forms that look fine in portrait often hide critical fields behind the on-screen keyboard.