My problem is that shield gets chewed through so fast that I don't know if I should go ham once it's down or wait for my team to lay in more poke.
I see a lot of top reins initiate once the shield is downbut I don't know if that's the best thing to do in pubs.
It all has to do with positioning. That's why you have to constantly be pushing forward towards the enemy Rein while your shield is still up. Keep the pressure up and make the enemy team respect you and give ground. If your shield is getting shredded, you need to take cover and let it recharge before you continue your advance. Like I said, it does recover faster if you don't let it get completely broken as well.
But if you aren't in the enemy Rein's face by the time your shield goes down for the push, you are likely going to go down quick. Going ham is based on the situation. If it's all squishies on the cart, go ham (esp if you have a healer with you). If you have a flanking Genji on, say Hollywood point A, and you hear his sword go off (assuming you haven't already lost 2-3 people)? Charge that point even if you have full barrier. The entirety of the enemy team
will turn to focus the genji. Your job there is to instigate a fight as well and give them multiple targets to try to divide focus upon. You have to learn how to play to the calls as rein.
It's easy to watch pros and see how easy they make it seem to go ham, but what you don't really realize is the timing and reasoning behind it all.
Until you start to get that timing down, though, I'd recommend just holding up the shield and constantly pushing and repositioning, and only going ham when there is something in your face.
This is largely anecdotal, at higher SRs, DVA barely ever gets any big plays.
At higher SR's all it will ever be is earthshatter, and I'm okay with that