by how much the overall damage in the game should go down? poll

A flat damage decrease would never work.

but i'll play along and go with the 20% option. If we take all champions individually, and lower their overall output by 20%, I think the game would feel more tactical and less "who shoots first" or "who counter-picks who".

It's not about flat damage, but about the scaling, free damage and penetration from runes reforged that is the cause of most damage creep, as well as defensive items being nerfed and defensive options in runes largely removed until recently, where the missing options returned at very underwhelming values.

A 2/0 and average farmed (no pro-level or Bronze-level farm, just average) Syndra at level 9 used to deal a decent chunk of damage with W(Ball)QE. This combo, when timed correctly, deals all of its damage at once. I have personally one-shot both enemy bot-laners in Season 8 with Electrocute and Q(Ball)WE, meaning only to chunk them a decent bit with a stun and maybe set them up for a kill. They both died instantly. I was level 9, had 2 kills and normal farm, as above. Normal build, no 8 Stack Mejai's for outrageous mid-game damage or anything like that. Syndra is a control mage with sustained damage and some burst potential, but her burst is largely tied to her ultimate and her base kit has no business one-shotting people, much less two people, unless she is insanely fed.

Damage has reached abominable levels. As I said, it's not about a flat damage reduction across the board (because at some level, tanks would become broken), but damage has probably reached nearly double what it used to be years ago. If I had to give a flat damage reduction, it would be in the realm of 35% - 55% so that damage falls more in line with what is used to be, where the checks and balances of design were at the healthiest.