I've been successful in getting OpenWRT 19.07.7 running on the ERX-SFP, following the information here: Step by step install openwrt onto edgerouter x? - #2 by pwned and the factory image from https://github.com/stman/OpenWRT-19.07.2-factory-tar-file-for-Ubiquiti-EdgeRouter-x.
As I understand the stman repo readme, the root problem is that the factory image is missing as the openwrt build fails as a result of a slightly-too-large kernel image. stman trimmed the kernel build and graciously provides a working factory tar file from which you can then sysupgrade to a stock openwrt image per the ToH directions. I had to force the sysupgrade as the stman image is for an ubnt-erx and not ubnt-erx-sfp, I switched to latter for the final install as that is the hardware I have.
I will try and update the wiki with this info a little later.