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.
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.