I wish there was a function on 20XX that repeatedly created edgeguarding scenarios for different matchups. Even one as simple as them recovering high/med/low and using different options would be great. I would practice edgeguarding all day lol.
The biggest thing is your jump positioning. You have to place yourself in the perfect position to be able to cover the options. You can then react to the illusion/phantasm while in mid air with uair or bair, and with fastfall (even dj with bair if you're fast, and jumping forward/toward center stage gives a few extra frames for the hitbox to come out) when necessary.
Reverse uair is still somehow a little bit underused. Dair covers sweetspot too, but has more L-cancel lag and requires you to be closer to the ledge than does reverse uair. Save stomp for covering dj sweetspots, as they are slightly harder to hit generally and dair reaches lower. Reverse uair all other sweetspot attempts. With proper jump positioning, you can bair on reaction when they don't sweetspot, and uair when they do. You have to focus on edgeguarding just as much as you would focus in a chaingrabbing situation. When reacting with uair to the sweetspot, respond to the earliest visual cue/sign of the side-b (before the *ting* nose). It's very possible, try it. If you miss a reverse uair you can often sh bair immediately and beat out LHDL unless they place the first laser really well.
Once you're effectively covering sweetspot, you can fake the reverse uair with an empty sh and just land and ftilt/jab their side-b. This is one way of getting them to do what you want them to do. Another way is to fulljump bair into falling uair for sweetspot to force them low.
Yet another way is to take ledge and ledgehop at the last second and low ftilt, ftilt, or jab. Most spacies will not side-b to ledge if you have ledge, although they sometimes like to call you out with up-b to ledge, in which case you have to react with roll. Taking ledge is also good if you know they can barely make it back onstage, as you can ledgehop grab them (even ledgedash grab), then chaingrab until they DI offstage (ftilt/aerial/dtilt/offstage uthrow trap/fthrow offstage). And there's also always INV ledgehop Tipman spike, but it often requires a read (at least you can immediately low ftilt after). Ledgehop uair regrab can bait an early decision out of them also, as can empty RLD.
In most scenarios, they have too many options to go for a read. Practice reacting and perfectly placing your jumps, and using fastfall when the situation calls for it. This works, it just takes practice.