There may be more to come on the minimum monthly spend but for now it seems that it only applies to those who signed up before 2021. Those who started this year and in the future still benefit from credit never expiring (unless terminated for inactivity).
Wi-Fi calling depends on the phone. I have just moved their SIM from a Moto G5 to a Pixel 4a and the service now works.
Maybe, I'm currently burning through the credit, 4g voice and wifi calling work fine in a Pixel 2 and a Redmi Note 9T, 5G data also works in the 9T.