As others I struggle with hardware trigger of ADCs.
My base was NXP's example adc_etc_hardware_trigger_conv. It works as expected.
Next I tried to do the same but use ADC2 instead of ADC1. I didn't get any interrupts. I used trigger 4 instead of trigger 0 and I added an interrupt handler for ADC_ETC_ERROR. I received periodically ADC_ETC_ERROR but no ADC_ETC_IRQ0/1 as expected.