A water change in that range is really more of a bioproduct of cleaning the tank more so than a mechanisms to impact the water directly.
Changing twice a week seems really excessive unless you have it automated. But that's just me!
That said, your tank being bare bottom, new in January, using all dry rock -- to me, that is the biggest reason why things are going sideways.
It takes some real time to get a bare bottom tank fully ramped up, especially if you started with dry everything.
I had similar experiences when I went barebottom where all the numbers were correct but stuff kept dying.
Not saying bare bottom doesn't work, it very obviously does work. But the ramp up time to a successful reef is longer.
Do you have a refugium? Any extra bio media like siporax or seachem matrix etc?
Do you do any bacteria in a bottle dosing like microbacter 7 etc?
Personally, I'd reduce the water changes. Get the doser dialed in. Stop dosing nitrates. Feed more.
And I'd leave it to people with more bare bottom experience.. but I'd try to get a live rock from an established reef with some nice sponges etc growing on it to help establish some real life in the rocks if you haven't already done that. Have to be careful there not to import a bunch of garbage like aptasia though.