As I said I had the same problem and ordered a USB board (witch include mic) on Aliexpress (https://www.aliexpress.com/item/DOOGEE-Y...).
Firstly if you're not confident with soldering and de-soldering you should consider buying one with vibrator and speaker as they are connected by soldered wires to the board.
To remove the back aluminium simply use a "mediator type" tool.
I recorded a little video of disassembly but I'm not a pro : https://photos.app.goo.gl/n15VVmt4CXSjsC...
Then unscrew the 4 screws and the usb board is "glued" to the plate with a double side sticker not easy to unglue. You should also take care of the little cable
Then unsolder vibrator and speaker and fix them to new board and mount all back.
I may have some other videos if you need.
My Y6 Max works now great ;)