Yikes! I've got a grandfather clock with a mechanical movement that does about 20 seconds per week. I'm glad you warned me about the rtc. I will forget about it for anything needing long term accuracy.On a random sampling of a very few Nextion displays (although I've used about 50 so far on my projects), the accuracy of the Nextion RTC is, IIRC, of the order of +/- 3 minutes per day...
...so I use an onboard (and genuine) DS3232 for RTC purposes which gives me an accuracy of around 2 seconds per week.
I've seen posts on other forums about the non-square pixels on the bigger displays. I guess x, or y scalling will fix this, but it is just one more thing to keep in your mind.