Pinedio stack is probably the best cheap option, as it has been fully reverse engineered, including the radios- no firmware blobs. Similar to m5 stack but riscV

Pinedio stack is probably the best cheap option, as it has been fully reverse engineered, including the radios- no firmware blobs. Similar to m5 stack but riscV core. For an OS you have nuttx, but for the memory size, Oberon ports (A2) might be feasible.