I just bought a 2nd hand rt-ac88u and have the same problem: not seeming to boot.
Flashed openwrt-22.03.3-bcm53xx-generic-asus_rt-ac88u-squashfs using the CFE miniWeb Server.
Version openwrt-22.03.2-bcm53xx-generic-asus_rt-ac88u-squashfs works fine (it boots).
I attached a serial console and made a log: https://pastebin.com/bMfNcq4t
Maybe someone understands what is going on. The last part:
iproc_nand 18028000.nand-controller: intfc status f0000000
iproc_nand 18028000.nand-controller: timeout waiting for command 0x1
seems to endlessly repeat.