I’m sure you used to be able to choose which NTP server you wanted to use by typing in the domains? Was this functionality removed from LuCI?

Fixed in the Turris OS 3.11.6, upcoming next version of Turris OS 4.0 and as well with Turris OS 5.0 (branch HBD, based on the cooking OpenWrt 19.07) and I shouldn’t forget also Turris OS 6.0, which is daily build on branches master of OpenWrt. Thank you for your reporting.