Will you be making your own silicon too? Because there’s nothing stopping the risc-v processor manufacturers from slipping in extra logic while making the die.
I haven't kept up with it, but OpenCores is a balwark against this type of thing. FPGAs, while not as efficient as fab silicon, AFAIK lets one implement CPUs, interconnects and peripherals without any predefined channels to target for subversion. The NSA or other boogeymen couldn't craft a backdoor for your FPGA CPU, since the FPGA is just a 'blank slate' until programmed so they have no idea even what to attack beforehand. The chip could be literally anything once programmed. FPGAs by design have to faithfully implement the basic gates, with no jiggery-pokery, otherwise it would be evident immediately that something was up. Right?
FPGAs are mostly proprietary products with proprietary technology inside. Many also have "hard" IP blocks for various things sometimes including a "hard" ARM based computer subsystem.
If you are getting one and flashing your own CPU to it it will be harder to attack, but definitely not impossible. There have been vulnerabilities in FPGAs before.
I tried this once in college, and even at a simplified scale, it was ridiculously difficult. The single course was half a semester's worth of units, just on its own.
Hopefully there will be several manufacturers to choose from, maybe even from countries that hasn't incorporated secret government spying through companies by law. It's not the manufacturers I'm afraid will incorporate back doors.