Welcome. Many around this forum like 20. I use 24, since it's far enough away from 2.4 wifi.
5 gHz wifi has no effect on Hubitat Zigbee.
I'd say to use 24 if you can. Some oddball devices will only join on 20.
You can absolutely start on HE on any channel, then change the channel. It's takes some time, but all devices should rejoin themselves after channel change.
See this thread
PS- a call to ST support can have them change your zigbee channel
