This is driving me insane

This plant looked fine when I got it from PLNTD about 1.5 years ago. They sell with a guarantee (that they don't seem to honor in any way..) that you don't need to re-pot for a year. After 3-4 months it started to look weepy and lame. I played with the watering schedule, fertilizer, lighting over 3-6 week iterations - no difference. Just a continuous decline.

I repotted it. I was root bound as hell. I put it in a looser soil in a bigger pot and even punched extra drainage into it. That will do it!

No. That will not do it. Continued to get worse. Tried different water sources and fertilizer. Nope. Different light. Nope. All the things. Treated with Neem for invisible pests. Nothing. My other plants, including orchids continue to thrive and this thing is a zombie.

What gives?!?!