I think 1450 EUR for a 16GB RK3588 is hard to justify. Is the Rockchip open to begin with?

I'd go for a framework using the Roma or CIX boards if I wanted to go for an "open hardware but not really" goal.

RK3288 was true open source. It boots with just u-boot and no blobs. DDR init was done by u-boot code and armv7 didn't require a TEE.

RK3588 is "almost" open. It boots with mainline u-boot and device tree from the Linux kernel, but needs two blobs: the DDR training blob and the trustzone blob (BL31). It can run without a TEE OS. I recently heard that the BL31 is now open source, but I didn't look into it. Mainline kernel has support for everything, including 3d (panthor driver) and video codecs acceleration (you find it in the v4l submenu). Mainline Mesa driver (panfrost) also works, but... not great - it stutters/freezes when I move the mouse.

other than maybe some RAM related blobs I believe the mnt stuff is completely open in both ways

framework stuff is generally neither open hardware nor open software, intel and AMD don't permit it