SPI, 8+ Digital Pots on 1 arduino.

Projects General Guidance
4

Yes, I'd go with 74HC138 or 74HC154 & MCP3208 ADCs.
3 address lines select 1 of 8 ADCs. Use 5-6 outputs, select 7 or 8 to not select any device.
Or use a 4th control line to not select any output.