It is possible to upgrade the boot loader from EdgeOS 2.x before flashing OpenWrt. I think it is worth following the instructions on the Ubiquiti website to do this if your ER-X came from the factory without it. It is much easier to recover a bricked ER-X back to factory firmware (and from there flash OpenWrt again) with the new boot loader. Perhaps water under your bridge at this point, but others may benefit.
If you set up VLANs to segregate IOT and guest interfaces on your home network, you might want to stay clear of the current OpenWrt snapshot and stick with 19.07.x stable for now. Snapshot has converted from swconfig to DSA to manage the switch, but it remains a work in progress with no Luci support yet. The physical ports were mapped back to the OEM factory defaults with the DSA conversion - so the reminder to move your cables to the right ports when going from factory to OpenWrt on an ER-X applies in reverse going from OpenWrt 19.07 to OpenWrt snapshot LOL. Additionally, if you want more ports to have access to the default lan interface, you'll need to change the config files to bridge them manually as they are not mapped to an interface by default in the current snapshot. It's been awhile, but as I recall, this too is consistent with the EdgeOS defaults.
I think you'll be happy with it. OpenWrt has worked really well on my ER-X as a home gateway (definitely better than Edge OS). Plenty of memory and flash (256MB each) to load it up with features, the CPU is just fast enough to handle SQM QoS on my 200/10 Mbps ISP connection, it is compact enough (downright tiny in fact!) to easily tuck inside my home telecom wall panel and it runs cool inside that small enclosed wall panel thanks to its low power draw.