My hopes of large volume fully open source systems died when I learned that beefy RISC V boards will ship with UEFI.
You can implement something else, riscv is a young isa
yup, RISC-V happily boots with just uboot
Yep!
You can run linux even on an entirely open source from hardware to software toolchain: https://github.com/litex-hub/linux-on-litex-vexriscv
Though the FPGA IC itself of course is not open, the bitstream generation is, and there are many fully open source hardware board designs, for example the orangecrab. With a Lattice 85K gate FPGA, you can get 4x 32bit riscv cores at 50Mhz or 1 64bit riscv rocket 64 bit core at 20Mhz