I was trying to install openwrt on a Linksys e8450 router, and saw something about downgrading 1.1.x to 1.0.x. I did not go through Linksys' sign up, just used their default password, and installed 1.0.x. it boots into 1.0.x but after giving it the default password I see a flash of the main screen and immediately get thrown back to the login prompt.
Can I do a tftp flash of openwrt, and if so, version of openwrt should I use?
I've seen this exact issue happen on some newer web browsers due to broken support for the quirks present in the firmware's web pages. You might want to try using a different browser, or maybe even an older 2020-2021 era release of a web browser if you can get a portable package or an older version. Using the web installation method is preferable and well tested, but other ways of performing installs do exist as well.
You can use TFTP via the factory bootloader, but the factory boot environment is quite different from that used by OpenWRT. If you use the U-Boot boot menu via the serial console, it'll expect the following parameters:
IP Address = 192.168.1.1
TFTP Server address = 192.168.1.254
Filename = lede-mediatek-mt7622-MTK-AX3200-MT7531-squashfs-sysupgrade.bin
Regardless of the method used, you should be able to install from the same OpenWRT image. If you are looking to install OpenWRT permanently (as in not going back to the stock firmware), then the wiki page for this device has links to the page with the OpenWRT UBI installer. The most recent stable UBI installer listed at the moment is v.1.0.2, which will start you with OpenWRT 22.03.x. From there, you would need to install the sysupgrade file for 23.05.4 to get to the current version.