I seem to have killed my bonsai (variegated ligustrum) after repotting it a few days ago. I used “bonsai focus” soil from a garden centre mixed 50:50 with a mix of akadama, pumice and lava rock. I kept it out of direct sun, tho my room can be a bit warm. I watered it throughly and it hadn’t dried out. It was a little ropey before I repotted it but definitely still alive. I did the bark test and it’s completely brown underneath, so pretty sure it’s dead.

A few of the finer roots came off when repotting, but not the main root mass. I don’t understand what I did wrong. Any advice would be appreciated!

What you did wrong is placing it inside. There is no such thing as an indoor tree.
If it isn’t freezing in your part of the world place it outside and hope, ligustrum is a tough species, there might be some life left somewhere. Water thoroughly with a garden hose or watering can untill water spills from the drainage hole underneath.

Outside, always. That species is very resilient. Put it outside, where it belongs

Foto 2 has signs of overwatering(root rot), and underwatering.

I did the bark test

Like 12 times? Don't damage tree's when there sick.

A few of the finer roots came off when repotting, but not the main root mass.

Care to clarify this? Fine roots do all the work, ticker roots do not do anything.
Do you have a picture of the roots when you did the repot?
Did you do an emergency repot because of signs of overwatering? Root rot?

It would've gone better if you placed it outside. Trees belong outside, especially that kind.

I don't have a picture of the roots. I repotted due to poor soil and drainage.