UART or I2C on NANOPI R1

I use NANOPI R1 in one of my projects. Now I need to develop a board to make a physical watchdog from NANOPI R1 itself and an RTC (the NANOPI R1 RTC does not work). I have two possibilities:

One is to use the already integrated UART, or use a USB - I2C converter.

Using the UART I would communicate the NANOPI with a 328P microcontroller, and from there I would collect the RTC information (with i2c between 328P and RTC), in addition to sending the frequent information to the 328P to keep the watchdog active.

Using the I2C, the RTC would be connected to the I2C BUS and I would do the direct query via NANOPI R1, as well as send the frequent information via I2C to the 328P or ATTINY85 (I can use this CI - it have I2C) to keep the watchdog active.

I want an opinion:

I would like to know how you would do that? Would it be using I2C or using UART? If they were going to use UART, I need the two ports available on NANOPI R1 (because I already use a Nextion HMI on a UART port).

So can i use UART_DBG too? What problems can I face?