I'm using this to resize the rootfs:
CONFIG_TARGET_ROOTFS_PARTSIZE=448
(it's snapshots default also)
My build sysupgrade code size is >80MB included the app Docker.
All the rest room of the eMMC, formated an ext4 overlay disk by cfdisk, and used by the docker.
Then I could run the native databse + native grafana + portainer Containers in the docker (>1GB code size).
When I do sysupgrade+keep Settings, all Containers in the docker, auto mounted and no need to re-install. All old data in the database still there, this is very important.
Now, my router (RAX3000M-emmc) = an openwrt router + a docker running any linux softwares. My separate linux server is no-use.