TP-Link Archer C60 v2.1 US - Flashing via serial, no input accepted

Hey all! I am trying to install OpenWRT on an Archer C60 v2.1 US I had laying around. I was stupid enough to update to the latest OEM firmware first, which judging from the wiki page for it, makes upgrading from the WebUI and TFTP, apparently, impossible (at least for the WebUI, I get an "Invalid File Type" error). I am now trying to go the serial way, I have soldered a 4-pin header in the PCB of the router, hooked up the TX of the PCB to my USB-To-TTL adapter's RX pin, RX to TX, and GND to GND. My adapter has two extra pins, one labeled VCC and the other 3V3. I left out the VCC (3.3V) pin from the PCB disconnected because if I connect it to my adapter's VCC pin and then try to turn on the router with the adapter already connected to the PC, not even the power LED will turn on. However, if I first turn on the router with this pin connected and THEN I plug the adapter to my computer, it does show the serial output just fine, just like it would if I didn't connect the VCC pin at all. The only significant difference I notice is that it makes my adapter's LEDs turn on without having to plug it to a USB port at all.
After that electrical weirdness, I can successfully see the UBoot logs in PuTTY. However, there is nothing that tells me that the router is successfully accepting any input, spamming tpl and enter before/during the "Autoboot in 1 seconds" message does nothing, just like pressing Enter when the "Please press Enter to activate this console." message shows up accomplishes nothing either.
I have no idea on how to proceed, I doubt it's my soldering what's wrong, it could have something to do with that VCC port but I also doubt that. I have disabled Flow Control from PuTTY, and I am sure my TTL adapter is the one at fault here because I've used it plenty of times before to talk to other chips (a PS3's syscon and a Raspberry Pi 1 to be more specific). I'd really appreciate any advice, either on downgrading to the 2016 firmware so that flashing from the WebUI is possible again, or any alternative methods. The wiki page for this device is pretty dead and doesn't seem to have an enough of a big community.

Turns out, the factory image for any 23.xx release is missing, presumably from build errors. Uploading the 22.0.3 factory image through the WebUI works just fine, afterwards I uploaded the latest 23 sysupgrade image through LuCI and that worked just fine. No need to use the serial port, yipee!
For anyone who arrives with the same problem as I, here is the factory image I am talking about