WD OoS only gives Marth 1 frame of advantage over simply unshielding. The main benefit of the WD OoS is that you can move more quickly during the 15 frames of jumpsquat + airdodge landing lag. I frequently WD OoS just fine with just the L-trigger, and the thing to keep in mind is that airdodging is done by clicking the trigger with a full press. If you are light shielding, you can simply keep L/R held while you jump OoS, and then push the trigger further down to airdodge. It is virtually equivalent to a regular WD when you're not shielding, but your trigger finger starts slightly depressed instead of completely off the trigger.
However, since full shielding is usually the preferred shield mid-game, you shouldn't just try to light shield everything (it'd be harder anyway). Instead, you just need to learn to raise your trigger finger slightly so that you are no longer holding down the button part, and then repress it after you jump OoS. So a simple breakdown of how you would do it is:
1. Hold shield (fully pressed).
2. Jump
3. Release the trigger just enough to unclick.
4. Repress the trigger so that it clicks.
This method is pretty difficult if you haven't been WDing for a long time, but even with the fastest jumpers in the game (Fox, Samus, Sheik, etc. who are all just 1 frame faster than Marth fyi) it is possible to do consistently. You really just need to focus on doing steps 3 and 4 during jumpsquat. Marth has 4 frames of jumpsquat, so you have about that many frames in which to unclick and click the trigger.
Another alternative is to use a separate trigger for shielding and WDing. I use the single trigger method when I shield normally, but when I decided to learn to PS projectiles and incorporate shield stopping into my game, I trigger tricked my R button and used that trigger instead. I still use L for the majority of my shielding, but because I WD with L, I am able to PS/shield stop and WD out VERY quickly, and VERY easily. I could probably do it very well with a single trigger, but I'd inevitably have to focus a little more to make sure I'm releasing the trigger at the right time and repressing it fast enough to get a good WD. I also enjoy the benefit of being able to easily L-cancel with L after shai dropping through platforms with R.
Hopefully that answers more questions that it raises. lol If you shield with R, I would just highly recommend trigger tricking your L-button now (unless you L-cancel with it) for PSing, and then you can simultaneously practice shield stopping/PSing as well as WDing OoS. You don't have to actually even be PSing at first. Just go to FD in Training Mode and dash into shield (with L), then WD out (with R). Then you can do the same to practice only using R for whenever you find yourself shielding regularly. Also, the reason I wouldn't advise trigger tricking the trigger you L-cancel with is because you will waste a tech attempt every time you L-cancel. If you haven't already, learn to L-cancel with a light trigger press so that if you get hit out of your SHFFL you can still tech. If you hard-press a trigger to L-cancel and get hit right after, you won't be able to tech (there is a 40 frame window allowed between tech attempts to prevent mashing for techs).