The Kendryte is pretty limited in many ways, not least that it's permanently limited to 8 MiB of DRAM (AFAIK).
The most interesting route is using an FPGA. There are many examples of running RISC-V softcores on FPGAs (for example, Berkeley's RocketChip) and many boards that support a decent amount of memory (even beyond 1 GiB).
I tried some of SiFive's evaluation cores. They limit you quite a bit unless you pay. Furthermore, many of the FPGAs worth their salt were quite expensive. The K210 in the MaixBit form is only $20, and it comes with a camera and LCD. For hobbyists, I think it's a good compromise.
I looked at the Xilinx FPGAs--the ones SiFive recommends, but they're quite pricey.
I'm not sure what's the status of programming these from Rust. I suppose that you will probably need to build your own toolchain that links the Seeedstudio libraries.
With Rust, you can do what I did with my OS. It can compile out of the box. The kendryte has a "BSP" (base support package) that is essentially a minimal OS, but it is written in C.
Well, FPGA are more flexible but cost more than a cheap microcontroller that is true. There are way too many good options, but to pick one with a fantastic community: ULX3S
