My first home computer was a TRS-80 with a Z80 processor and Zilog promissed to bring out the Z800 which intrigued me, but later they did make the Z180 and I recently bought some of them on AliExpres. The schematic that I make public today isn't finished yet but gives a good idea of what I'm trying to do. I still want to add two micro-SD-card-interfaces and two USB-connectors, one for a master device (like a PC) and one for slaves like keyboards and mice.

The Z180 can address 1 megabyte of memory so I have two half-megabyte RAMs but it has to startup in ROM of course. So when the computer starts up, the first memory-bank of 64 kbyte has ROM and the other 15 banks have RAM. The code in the ROM can copy itself to RAM and then set bit7 of the latch (74HC574) which removes the ROM and maps the first 64 Kbyte of RAM into the first memory bank. If you think that 512 kbyte of RAM is enough for you, you can leave out the first RAM-chip and perhaps the latch.