The number of bitflips in your log capture is concerning. It suggests your router's flash chip may be of questionable quality, so you're definitely doing the right thing by moving to 24.10 with UBI. It's quite possible that you did nothing wrong at all when you initially tried to run the installer. If the data was already corrupted but it hadn't yet been recognized as bad, that could on its own cause the installer to fail. Regardless of the initial cause, the end result is the same.
Since neither preloader can find the next step of the boot chain, it's pretty safe to say that the data on flash (aside from the newly installed BL2 / Preloader) is no longer viable. Recovery is going to require a complete reload. The good news is that you have had success with mtk_uartboot and are able to access the U-Boot boot menu through it, so you're already halfway there.
The above post contains instructions for complete recovery of the device from situations just like this. I definitely recommend using 24.10, since that puts nearly everything into UBI and so the data will be more resistant to bit-rot going forward.