buzz is everywhere, more so on the E,A,D strings
Sounds like the neck is too flat.
relief is good, the low E has a tiny bit of room on the 8th/9th fret
What's "a tiny bit"? Measure.
Should be in the .3-.5 mm area. I'm willing to bet yours will be lower than 0.3 if you measure it. If it is, you're pushing the boundaries of a "low" setup.
I did go up and down the fret board with a fret rocker and there are some high frets but nothing too crazy.
Again, measure. An unusually tall fret further up the board on a neck set up to be flatter could easily cause a lot of fret buzz every time you fret and pick a note below that point. It will take patience and some investigation, but if you check the notes where you hear it most commonly then find there's little to no clearance under the string with your feeler gauge, you can narrow down the offending fret position.
All I want to do is lower the action to something in the ballpark of 1.5mm on the Low E.
That's exceptionally low action for a guitar of this type. Remember, you're not working with Ibanez's highest craftsmanship. It can make a difference. Trying to eek out the lowest setup possible on a guitar of this kind might be asking a bit much. That's why I'm suggesting you break out feeler gauges and measure, adjust, and scientifically chip-away at the problems. I just don't know that 1.5-mm action is going to be achievable with a perfectly clear tone here.
It's also damn hard to give advice on this sort of thing without putting my hands on the thing. There's more you can check I haven't even mentioned yet, typing it's the hard part.