I am mostly using EspressoBin v7 with eMMC of 4Gb and ULTRA variant with 8Gb eMMC.
Both firmware from standard 21.02.0 in ext4 have only 2 partitions, first, very small for /boot (boot.scr, FDT file and kernel image) and a second one, small also for / (rootfs).
I always get a cloned ext4 rootfs to handle the rest of the eMMC in a third partition.
I am now testing with a f2fs overlay instead.
This third partition, extroot or overlay, is needed to get full use of the eMMC storage.
It also help of stability in case of crash, hang or power off !
But it is only possible with manual steps now.
These manual steps are needed after a first install but also at any sysupgrade.
The MTD likes devices do not need these, because the partition map is "better" handled, more frozen.
Another partition map may fit other use case, as a fourth one for persistent data, or another usage I have not think of...
I was thinking of tweaking the default ext4 firmware with a third partition map which handle the full size of the eMMC.
Or a small third partition map which will be resized at first boot to the full size of the storage.
Or a third partition map which fits half the eMMC and a fourth one for persistent data, which fits the end half part of the eMMC.
@vgaetera already helps a lot to get an end-user friendly manageable OpenWrt, with opkg and hotplug extras, and I already get all my needs for EspressoBin boards in Request for Best Practice for eMMC devices
Theses extras tweaks from @vgaetera with yours from Custom Online Image Builder and Upgrade Server may help OpenWrt to get in another step !
May be some parts need or will be upstreamed, like @vgaetera extras in a package, or device map in official image, or boot script multi devices handling in master, or more...
Hope only these ideas are not fully insane and get OpenWrt using more simple !