ur forgetting armor pen in ur calcs
also, what ur proposing would buff an adc's late game too much like vox1st said.
with armor pen calculated in, 1k damage would be around 2-3 auto atks from a late game adc with ie, pd, and lw, which would take a little over a second to do.
as for the shield strength, the only shield users i can think of that will be tanky would be nautilus and shen.
naut's shield at lv 5 hits 280, and without it, he's pretty much sitting there in team fights since he won't have his shield passive up.
and late game, a 280 shield, even with resistances, isn't much, since ur trying to tank 5 ppl throwing auto atks, aoe spells, etc at u
as for shen, his shield at lv 5 hits 250
i don't think there's anyone with a 300 damage shield that builds tanky unless it's a lux shield on her teammate
tldr, shields r fine as they are right now, otherwise they would all need a massive buff if u propose true damage on them