Thanks for the suggestion, Mark. I read through that thread and tried to gleam relevant info, but much of it seemed to be openwrt specific stuff so I didn't make progress in that regard.
I ended up getting Ubuntu to boot. I'll be honest, I'm really not sure what I did different this time. I started fresh, re-flashed U-Boot, set environment variables back to default, saved, rebooted, entered each U-Boot command again, reflashed my SD card, and was greeted with the familiar Linux boot messages. I setup a user account and logged in.
Now, after a few seconds, I see error messages like "BUG: Bad page state in process khugepaged". Now, Ubuntu is failing to load up upon every boot. If I re-flash the SD card, it seems to get further in the process and boot successfully. After a few mins of it just sitting there, I start seeing error messages popping up once again. What does this mean?
P.s. I've been using linux casually for years, but am not deeply familiar with the ins and outs, so forgive me if there is an obvious solution I'm not seeing here.