You're not SOL, but you'll need to go through a few extra steps to get the router working again. Whether caused by OKD or due to a broken flash, your router indeed cannot load the FIP.
For full recovery, go to the main topic for the e8450/RT3200 and look for the post marked 'hard recovery instructions'. That contains all of the steps necessary for restoring all of the flash data to the layout used by the 23.x version (BL2, FIP, and Factory in MTD). Depending on what has been corrupted, you may need to restore a previously taken backup image of the factory partition. You'll know this because U-Boot will immediately complain about not having a MAC (ethernet) address, and your 5GHz radio will not work (it won't even show as a device).
Re-writing the FIP is easily done through the U-Boot console. The big trick is getting there. After using mtk_uartboot, you get 3 seconds from the time the FIP is received by the router to the time the router stops waiting for user input and starts trying to boot. If your terminal program is too slow to load (PuTTY, for instance), you won't get a chance to do anything. If you can't speed up the load time for the terminal program, you can try the following FIP with mtk_uartboot: https://github.com/grauerfuchs/owrt_device_support/raw/refs/heads/main/linksys-e8450-foxed-for-openwrt-23.05-noinit.fip This particular FIP was compiled with longer access windows and a few additional features intended to make recovery easier, such as dropping right into the U-Boot console rather than automatically trying to boot. It is also built to be 23.x compatible, so it will not work with the all-in-UBI (24.x or snapshot) firmware.
For recovery purposes, you do want to use an official release for the process. The instructions listed in the post are very specific about versions. Don't make substitutions. You can later go back to your own custom image for the main running firmware if you wish, but the boot chain and recovery volume should always be the official ones unless you're willing to deal with issues and a complete lack of support from anyone. Custom images present too many variables and too many potential differences, so the general policy is to support only official images.
As for corrupted U-Boot environment, it won't prevent boot. It will only force the router to use the built-in default values. Since most people never change the U-Boot environment, this isn't an issue.
If you're in a working recovery image without any issues on flash, you can use the UBI installer image. If you have problems on flash (and it sounds like you do), do not try to run the installer. It will most likely stop and fail out because it can't find the data it needs.