Building 800x800x800 big 3Dprinter build/bed questions

Alright alright, before i get started. I know this is big, and its gonna be really challenging. Im studying electric engineering, so i have a lot of tools at my disposal. I'm looking for advice and discussion on how to progress in this project.

So the printer im looking to build is a core-XY with a fixed bed. I choose this because of the size and weight of the print bed. However im not sure if this is the right choice, i just believe it might be cheaper, since a non fixed bed would probably need 4x nema 17 motor, to move the bed in z axis. Also i believe a fixed bed would make leveling the bed quite easy, as it would always be level. The XY system could then automatically level itself to the bed. Is this the right way to do so, im not sure?

The print bed is probably the thing im wondering about the most. I want to print PLA mostly, and therefore want the bed to be able to hit around 60 celcius. However 800x800 is quite a large area to heat up. So my idea is to take 4x 400x400mm thermal heated beds from creality (link below), screw those 4 down in the frame of the bed except in the 4 corners of the bed frame. In the 4 corners of the bed frame i would then screw down a 800x800 glass bed. Then on the glass i would put adhesive layer and stick PEI on top of it. Pros and cons on this? Also would i need something between the bed frame and the heated beds?

I know there is alot of ways to design the bed. Just thinking if my idea is possible. I have also considered aluminium 5mm thick with silicone 4x heat pads underneath, but im very unsertain with this method, with my lack of knowledge of this type of bed, also the overall cost of the project would be much much higher.

I'm highly inspired by the Bigrep one printer. I'm very new in the 3D printer community i do own a 3D printer, just wanted to build a big printer for school use. I hope to get great advice! Thank you so much if you've read this far...

Edit - I should probably note that im choosing heated bed, too avoid the hassel of hairspray, glue, tape and what not.
- Creality 3D® 12V 180W 400 * 400 * 3mm Termisk Opvarmet Bed Hotbed Til 3D Printer Kontorudstyr from Computere og kontor on banggood.com