I am doing the same thing and I imagine you have seen my topic Connecting a Wacom I2C touchscreen to the Framework 13 motherboard (prototype). I can elaborate on why I ultimately chose the display I did so you can at least some some research time. For a DIY 2 in 1, there are 2 types of viable displays.
A usb drawing tablet is probably the simplest choice as it only needs 1 usb c cable to work and you do not need to interface with the eDP connector. The down side is that it will take up 1/4 of the IO and you would have to disassemble it to make it fit in the display. I was looking into the Asus ProArt Display PA169CDV before I decided that it was not the right option for me.
The other option is a display form a 2 in 1 laptop. This has the benefit of being able to interface with the eDP connector at the cost of additional work. As for as i know there are no 2 in 1 devices with stylus support that have a touch controller that connect over usb. I looked at displays primarily from Microsoft, Dell, and Lenovo laptops, as they all made a range of laptops with 4k displays with stylus support. I know that, despite using MPP, both the dell that i was looking at uses I2C for the touchscreen and older surface devices use I2C and newer ones uses “a rather special system”. Levovo devices also use I2C of course. In my research I never found a display that used SPI.
I ended up going with a display from a lenovo yoga 720-15ikb because I needed all 4 usb c ports on the motherboard, I already had the laptop, I can buy another off of ebay for the final device when i need (the dell display especially were very hard to come by for sale), and because i am already invested in the wacom AES ecosystem.
I think that a disassembled usb tablet may make the most sense for your project. First of all, if you do get a SPI display, i do not know of any easy way to get the OS to recognize it. The drivers designed for it will almost certainly not interact with a usb-spi bridge and there are no SPI exposed pins on the motherboard to connect it to. An I2C display will come with the same requirement of modifying the ACPI table to get it to work so, if that is a no go, i think a usb tablet is the only option remaining.
One thing you may be able to do is to disassemble the usb tablet and then disconnect its display from the controller pcb and then connect it to the eDP connector directly. This may then allow you to connect the tablets controller to one of the frameworks internal usb 2 ports (ether on the eDP connector [no idea if it works], webcam connector, or trackpad connector), allowing you too still use all 4 usb c ports on the motherboard. I was never able to find tear down photos of drawing tablets so I have no idea if the displays could be connected to the eDP connector and I have no idea if the controllers in the displays would work if only connected to usb 2 without a display signal, but it may be something to look into.
Whatever you do, keep posting updates as i would love to see what path you choose and how the project turns out!