Hi - two questions:

1) Can you use any NTP server? I was thinking of using time.apple.com and time.google.com.

2) In order for the automatic switch from DST to GMT (and vice versa), does the DST checkbox need to be enabled? Or does the advanced settings turn it on/off based on how they are set?

I use stratum 2 servers from the likes of here: https://timetoolsltd.com/information/public-ntp-server/uk/

find one "close" to you and you should be fine. Strathclyde and Manchester Uni are my gotos for many years.