This is likely a result of the very same issue (OKD) as it continues to progress. The good news: The cause of OKD has been found and resolved! The bad news: The fip might actually have bad data at this point, so it's time to properly replace it.
Since you already have a USB to UART adapter and have used mtk_uartboot already, you have some experience in what to do. You're going to need to follow some of the instructions from the below post:
Based on your above post, it looks like you're on the release version of OpenWRT and not the snapshot. If you're on snapshot, please say so. The post and instructions are not compatible with the snapshot and would cause loss of critical data. Again, please proceed only if you are running the stable, release version 23.05.4 or older.
From the linked post, you're going to need to follow the steps for replacing the preloader (BL2) and the fip. You will not need to replace or restore any other components as long as you are running the stable firmware. However, once the router is properly booted up, please make sure you've taken a backup of your factory partition.