White LFT? What size trowel notch? Doesn’t look like you’re getting nearly enough coverage and some of the heat wires are telegraphing. Plus, marble is translucent, and the deeper dark veining shows up when wet. You do have the cheater hooked up to the heat wire circuit to make sure you’re not nicking any wires?

Aren't you supposed to seal Marble tile before install?

The trowel size is way too small and you aren’t filling in the Ditra enough. Plus your likely using old thinset without enough bond if you’re moving that slow. You have to use more and fresher mud and move much faster. Burnish it in. Backbutter. And use a bigger grout line.

You also have to test the wire before you set one thing. And it has to be hooked up to a continuity tester during the entirety of the install so that will sound an alarm if the circuit is damaged. That is an absolute MUST.

You need a Pro. This has Fail all over it. I’d have to question if the Ditra and wire was even installed correctly at this point. What thinset is under it, and what is the substrate under it? Who planned the wire spacing loops and path?

I'd only use a Schluter modified mortar or an unmodified as they specify.

Worse and worse.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer 

Definitely not enough mortar. It is easy to see. I would be using a 3/4" trowel not 3/8" Hope you have 2 layers of plywood under the heat mat. Single layer is unacceptable for stone. Not criticizing simply pointing out facts.

"Not criticizing simply pointing out facts." Thanks @Creative Tile, I appreciate the advice. I am a software engineer, tiling is alien to me, trying to learn. All points taken.

https://www.tcnatile.com/products-and-services/publications/218-english-publications/188-handbook/957-2018-tcna-handbook-for-ceramic-glass-and-stone-tile-installation.html