The wiki page for Linksys EA6350 v4 at https://openwrt.org/toh/linksys/ea6350_v4 says the following:
"Upgrading OpenWrt
The advice for other Linksys dual partition routers, is to revert to booting Linksys stock firmware from the other partition (A).
Then install the new OpenWrt squashfs-factory image using Linksys GUI, as described earlier in this wiki page.
If you were to use OpenWrt to install the new squashfs-sysupgrade, there is a risk of overwriting the Linksys stock firmware on the other partition with other Linksys routers such as the earlier EA6350 v3 and EA8300. It is not known at this time whether this behaviour applies to the EA6350 v4."
I checked the page for the EA6350 v3 which is at https://openwrt.org/toh/linksys/ea6350_v3 and it says the following:
" Upgrading OpenWrt
Prior to OpenWrt 22.03.3, the recommended advice for this and other Linksys dual partition routers, is to revert to booting Linksys stock firmware from the other partition. Use the Recovery (Automatic) method described later in this wiki page.
Then install the new OpenWrt squashfs-factory image using Linksys GUI, as described earlier in this wiki page.
If you were to use OpenWrt to install the newer squashfs-sysupgrade image, the original Linksys stock firmware stored on the other partition will be overwritten. Both partitions will contain OpenWrt. Fortunately, it is though possible to restore Linksys stock firmware using the 'Back to Stock' procedure described later in this wiki page.
As it is now possible to revert to Linksys OEM firmware provided OpenWrt 23.03.5+ is installed, the normal method of upgrading OpenWrt is recommended:
Recommend to use the latest stable releases. Note the very development snapshots do not include LuCI."
So, for the v3 at least, there is apparently a way to revert to the OEM firmware even if both partitions have OpenWrt installed on them. And hence, the recommended upgrade procedure is "sysupgrade". I'm assuming this means that the upgrade can be done through OpenWrt and it is no longer necessary to boot from the partition containing the OEM firmware and doing an initial install.
My question is if it is safe to use the Attended Sysupgrade method to keep up to date on the upgrades? Or, should I continue booting to the OEM firmware and do it from there?
I would really like to avoid using the OEM firmware at all. It is really terrible. I'm not sure why, but, I have had a lot of trouble getting the OpenWrt firmware installed using the OEM firmware update function. The copy of the firmware to the flash appears to be unreliable. Sometimes it can take as many as 20 attempts or more to get it on there. On the other hand, overwriting the OpenWrt firmware with the OEM firmware seems to always work. I almost have to wonder if Linksys is doing something fishy trying to makeit difficult to install non-stock firmware.
If I don't care about ever reverting to the stock firmware, does it even matter? It sure would be nice to be able to just use the LUCI function to keep things up to date.