Aside from the wifi SoC firmware and components tied to the network swtich, there was one low-level precompiled file that was changed in 41535, which seemed rather random to me. I just downgraded it to the previous version, and so far this version seems to properly allow other images to be flashed on top of it.
Anyone with an RT-AX88U and the time to do some tests, please try the RT-AX88U beta 4 image uploaded to
https://www.asuswrt-merlin.net/test-builds/ .
1- Make sure you have a local copy of the beta 3 release
2- Make sure you have either Asus' Firmware Recovery Tool, or a TFTP client (personally I use FTP Rush, but there might be more intuitive clients available)
3- Make sure nobody will care if it takes your network down for a few minutes
Please test the following:
1) Flash RT-AX88U_386.1_beta4-gb9175c6a96_cferom_ubi.w
2) After the reboot, try re-flashing it a second time, see if it fails
3) Try flashing down to beta 3, checking again if it fails
If these tests succeed, then it means at least the firmware flashing issue will be resolved. No clue about the Trend Micro-related issues however, I haven't had time to investigate that (one crisis at a time...)