Matcha Flashing the boot.img from inside the zip is exactly what I was recommending you to do.
The main change Scott made to recovery was he added the ability to use ADB from inside recovery. He did not make sideloading update zips possible, because recovery usually checks the signature verification of zip files.
However, since the zip here is official TCL, it may work to sideload it (likely has tcl signature). You can try to run from recovery:
adb reboot sideload
adb sideload update.zip ##replace with the zip filename
It's possible flashing Zips after rooting can have bad effects, although I don't think you'll have an issue because you already flashed back the stock boot image, and the zip itself had a boot.img. But I never tried this and I don't take responsibility for anything that might happen, although I expect it to go smoothly.
You didn't do anything else besides flashing Magisk's boot.img? The only other suggestion I can think of is doing a factory reset (via recovery), and turning on the phone without a sim card. But that's a random guess. If you flash back ALL the stock partitions (in other words, any partition you modified), you can also try fastboot flashing lock to relock the bootloader.
