You can apparently use the open source yosys/nxtpnr tools with the tang nano 9k, but, unless something has changed recently, nxtpnr doesn't work with the 20K ye

You can apparently use the open source yosys/nxtpnr tools with the tang nano 9k, but, unless something has changed recently, nxtpnr doesn't work with the 20K yet. However, I found the Gowin tools to work reasonably well (and definitely way less bloated than the Xilinx & Altera tools.)