I dont know about mainland China using the seven luminaries as im an American born Hong Konger but as to the reason for me wanting to use it, it basically just boils down to "I like it" / I watch too much anime.
You talked about "Perhaps related is getting dates to display like 19 June 2024, June 19 2024 or 2024/06/19 without changing your region?" Could you leave a link to where I can find that setting specifically, I think ive seen that before but I cant find it at the moment.
Im currently using Traditional ZH(HK) language setting, which uses the date display system as the (for lack of better words) "Chinese system", where dates are listed like "6月19日, 週三", however I would like to use the Seven Luminaries system (https://en.wikipedia.org/wiki/Names_of_the_days_of_the_week#East_Asian_tradition), available on the JP language setting, which displays date like "6月19日(水)". I do not know Japanese but would like to use this date system whilst using Traditional ZH(HK) language setting. I have tried messing with settings in the Samsung Calendar app as well as messing with the settings of the phone itself but I cant figure it out.
I think I have done a pretty good job at describing what im asking about, but if not, I have also asked about it on Google support where I also included pictures: Change date display system on phone - Android Community (google.com)
Thanks for the help!