How much of a difference does cutting phone use before bed actually make?

More than the blue light explanation suggests. The screen's light does shift circadian timing somewhat, but the larger effect is content — messages, news and social feeds are arousing, and arousal is what delays sleep onset. Trials of simply removing the phone from the bedroom often show better results than blue light filters. Practical version: charge the phone outside the room, use a physical alarm clock, and give yourself a thirty to sixty minute gap before bed. Most people who try this for two weeks report falling asleep faster, and it costs nothing to test.