no high precision needs
What I can think of is that a low-cost ADC(TLA2022)+ low-cost digital isolator(2 CH) is used to measure the voltage after the 310V is reduced by the voltage dividing resistor.
The ADC with buffer and reference, usually Δ Σ architecture, two wires for I2C interface, so only 2-channel digital isolator is required
Any other suitable solution?