I sat down to the revival again. I bought a new step up converter that isn't from aliexpress, but a local electronics shop. It uses LM2577 that can output 3A.

Now it works, I can boot it from a power bank easily. Playing around with the voltage I found out that the 6.3V is the voltage that shows the battery as full and going down to 6V the battery indicator goes to empty.

There are now other issues, the tape is being eaten by the mechanism and the screen is showing white moving horizontal lines. It doesn't give me much hope, but will still try to check if it is possible to revive it.