@User: A few years back I added general-purpose LUT devices to XSPICE that allow any standard cell library to be implemented in ngspice; you just need the truth tables. I have scripts in qflow that use a liberty file to look up the function for each cell and generate the corresponding truth table, and writes the corresponding XSPICE circuit using the LUT devices. The script is "spi2xspice" and as long as you have a valid SPICE netlist with the standard cells in it, it will generate a valid XSPICE netlist from it.

I tried to find some more info and found the mailing list post https://sourceforge.net/p/ngspice/mailman/ngspice-devel/thread/57BF4C3D.7020909%40opencircuitdesign.com/#msg35316028 has this since been merged and released? Might play with it a bit later. That should take care of the "retarget Sky130" usecase. I'll think a bit about the "mixed signal design" case. I'm sure d_lut will allow more efficient yosys synthesis than NAND gates etc.