I flashed my original firmware v.2 & u-boot 3.0 to OpenWRT 21.02 snapshot today. I first tried the original method but it failed so I then tried your method with positive result.
Used console cable connection.
Actually it works so smooth I would have no problem seeing this method as formal install method if the other one doesn’t work.
The only thing I changed was to shorten the filenames a lot because PuTTY didn’t manage well with the longest lines of text.
I used the instructions above, had to copy image on EdgeOS to eMMC (uboot is version 3 on my router). After installing snapshot version (USB+terminal cable), I "upgraded" it to May 30 rc2 version using Sysupgrade. I'm more general purpose user than developer and more of GUI than CLI guy, but without these instructions it would have been impossible to get it up and running.
For me, reasons for choosing OpenWRT and EdgeRouter was 4G/5G wireless internet connection. I do have ZTE MC801A modem which has powerful hardware, but quite sad firmware. My ISP has CG-NAT on 4G, public IPv4 costs extra, public IPv6 is free, /64 prefix without PD. So I need something that can handle this IPv6 and is well documented, and I'll use ZTE as bridge. I can do basic things with Cisco IOS, but IPv6/64 without PD seems to be such new thing, that there aren't too many examples on Cisco side, therefore using OpenWRT. For hardware side, EdgeRouter 4 seems very powerful (I do have C897 and I'm used to snappy hardware in metal enclosure!) and is reasonably priced.
Thanks for damex and other people contributing for ER4-version!
soon after device was implemented - there happened to be a bug (not directly related to edgerouter 4) regarding not having a usb storage available for initramfs. it should be fixed by now and if it is still not - please report back so it could be checked even further.