There is funding for development of an open source GPU IP going on, based on OpenPOWER ISA (IBM POWER PC, which is open source as well by now with a lot of stuff associated with it like interconnects and whatnot being open sourced as well) afaik and being publicly funded by NLnet. For further information, check out https://libre-soc.org. I think i remember that there are RISC-V based efforts, too.
Mali GPUs are ARM IP and licensable.
About UEFI, of course, if you throw money and devs at it, there is a solution. AMD’s ARM based A1100 Opteron server cpus worked pretty much like x86 cpus apart from the instruction set and Ampere and Solid-Run probide UEFIs as well. That was the reason for my budget argument originally. Alternatively you throw open source devs at reverse engineering the living spit out of Qualcomm SOCs by airdropping large numbers of devices into the dev community or hope for Samsung (Exynos 1000 with AMD RDNA2 Graphics IP) to chose a more generic approach AND making them available for other companies. Things move into an acceptable direction at least. Things could be worse but for now the ecosystem just can’t provide what we need for all the previous reasons, sadly. Generic ARM (company) IP is well supported in terms of open source drivers btw, so you could maybe get a generic SOC up and running, if you license the full package and order it from TSMC yourself xD