On my R5S I installed a 1TB SSD through this manual https://wiki.friendlyelec.com/wiki/index.php/NanoPi_R5S#Connect_NVME_SSD_High_Speed_Hard_Disk and configured Docker (in the FriendlyWRT Docker GUI) to use /media/nvme/docker as home directory. This actually works for smaller containers, no problem....

4GB should be absolutely fine for HA.

Did you manage to install Docker? How did you overcome the problem with OverlayFS? Which image did you use and where (MMC, SD, SSD) is it running?

Currently I have the eMMC USB Debian Bullseye Core image installed on my R5S and it is running good. Now I wanted to install HA which requires Docker which requires ext4 filesystem and not OverlayFS. So I read this article https://wiki.friendlyelec.com/wiki/index.php/How_to_Install_Docker_on_Debian ...