Little puddin, I can't tell you how much I used to struggle against the withdraw approach.
The first thing to take into account is trying to read the withdraw.
Study your opponent and try to recognize when they like to withdraw. I pretty much know when the squirtles I play against are going to withdraw due to their spacing, and when you can predict the withdraw its a lot easier to deal with.
I used to shield the withdraw all the time, but like you said they'll just b-air you and repeat the shield pressure.
Then I thought out of shield options! At the time I was playing link a lot and I'd try to up-b oos but it was too slow =\ usually I'd just get b-aired anyways. Looking at the characters you play though, this strategy might work with samus cause her faster up-b (and possible invincible frames on start up? Not sure about that but read it somewhere.) Anyways I don't play too much samus so I've never tried this.
So eventually I stopped shielding the withdraw and started short hopping over it instead! Withdraw can no longer be turned around or jumped out of so once they pass you they either have to withdraw all the way off the stage or eat the laggy ending animation. If they opt to withdraw off the stage, bam, free edge guard opportunity. If they come out of withdraw on stage you can punish the animation by a) simply using an aerial if they pop out close to you or b) wave landing and chasing them down if they pop out a bit farther away.
There are a few easy character specific counters I've found as well.
Bowser with his crouch armor can just CC and down tilt withdraw all day.
As Mario, if you can read the withdraw its not too hard to time a down smash.
Falcon or Gannon, short hop d-air, follow up with free f-air
As Roy or Marth it's not too hard to space a WD back f-smash.
Anyways if you do still find yourself sheilding by reaction try WD OOS away from them right after the withdraw hits b4 they b-air.
Hope this helped.
I'm curious to see how other ppl are dealing with it.