I'm in a similar jam. I currently use an X201 Tablet, I refuse to move to a newer ThinkPad as the build quality really begins to suffer, and I'll be relegated to a 16:9 display.

I'm waiting for an announcement about the Raspberry Pi CM4, if they offer a model with 8GB of RAM, I'm very tempted to look into designing a carrier board that can replace my ThinkPad's motherboard. It will need an active DSI to LVDS converter to drive the display, and a small microcontroller to act as an embedded controller to read keyboard scancodes, trackpoint input and talk to the battery.