I'm asking this, because I am confused, because on the device page for my router - https://openwrt.org/toh/linksys/ea7300_v2
it shows the latest available as 23.05.4
The firmware selector shows the latest versions as it's picked up from the build agents, so the versions that shows up when searching for your device is the supported ones. The wiki is manually curated and not necessarily up-to-date for every device, I know that @RaylynnKnight has done a mega job updating the wiki and is probably still doing it now that 23.05.5 was released, so big kudos to them.
From the basic perspective of "Is the firmware selector a safe place to get upgrades?", the answer is generally "yes".
You might want to search the forum for a recent messages about problems with the firmware selector and it is definitely appropriate to read and understand all the relevant info in the release notes and "known problems" list and any other relevant info available in and via the release announcement in the "Release and security announcements" category of this forum.
Yes, should be fine. In some rare cases the build system has built firmwares for devices in preparation for a new stable release which compiles fine but then causes issue when running (such as the switch on Netgear R8000 and Linksys EA9200 not working, see OpenWrt 23.05.0 - First stable release), but as long as you check known issues (see post above) it should be fine; I've personally never seen issues like this between minor stable versions.
Your device also implements a dual partition layout, so even if the flash goes bad in some way you can swap to a known working firmware on the other partition, so it's basically unbrickable, which is a nice bonus.
You can't use a sysupgrade for this device. You must boot back to Linksys on partition 2, flash an OpenWrt factory image on partition 1, than restore settings
