I did the install of the latest armbian build and the device is now rock solid, no reboots after 18 hours - compared to between one and four hours to failure before the upgrade.
For the benefit of others who wish to repeat the process here are my install steps. I used a fairly powerful debian buster desktop to do the build but it still took some time, maybe half an hour including download time? I chose to only build the kernel elements by answering the prompts from the build script compile.sh
cd /usr/src/
git clone --depth 1 https://github.com/armbian/build
mv build build-armbian
cd build-armbian/
./compile.sh docker
cd /usr/src/build-armbian/output/debs
scp *.deb root@192.168.0.1:~
ssh root@192.168.0.1
dpkg -i armbian-firmware-full_21.02.0-trunk_all.deb linux-dtb-current-rockchip64_21.02.0-trunk_arm64.deb linux-image-current-rockchip64_21.02.0-trunk_arm64.deb linux-u-boot-current-nanopi-r2s_21.02.0-trunk_arm64.deb
systemctl reboot