I have both R4S and R5S.
I've tried FriendlyWrt and I did not like it (it comes bloated with too many packages). I also tried to build FriendlyWrt and setting up the build environment was a mess (nothing compared to the clean and self-contained OpenWrt build).
So I'm using the official OpenWrt on the R4S, and with the R5S I'm using OpenWrt builds I'm doing from https://github.com/mj22226/openwrt (clean OpenWrt, much better than FriendlyWrt).
I've already asked @mj82 (developer of the above repository) why R5S support has not yet been merged into official OpenWrt. His answer is here:
On a side note: DietPi just released a new version which is now supporting NanoPi R6S: