Because system upgrades flash an image to the raw flash storage just like your new installation did, wiping the partition table and filesystem etc, you lose any data in the extended area as well and you need to resize the filesystem every time and restore all your data from backups. I chose to use a ext4 filesystem on a usb flash drive and mount it. It usable on OpenWrt and my linux computer.
After a system upgrade on the sd card, I restore my config, reboot and all works as before with access to the data on the usb drive.
I write the statistics data files to the usb drive so I have long term stats etc. and reduce wear on the sd card minimizing the risk of a system failure due to media going bad. I also keep config backups and other data on the usb storage as well as on my pc.
Using the "Customize installed packages and/or first boot script" at https://firmware-selector.openwrt.org/?version=23.05.3&target=rockchip%2Farmv8&id=friendlyarm_nanopi-r4s lets you build an image with your added packages.
@efahl has an excellent script to automate the creation of the list of what is installed. See this topic for more info: How to upgrade correctly? (Nanopi R4S)
I have expanded the upper writable portion of the squashfs and can assist in that if you want to learn that process. Let me know.