First off, wireguard requires monotonic time (always goes forward), not correct time. All that matters is that time marches forward. OpenWRT already has a decent mechanism for getting a reasonably decent time in the not-too-distant past at boot.
Secondly, the lack of an RTC is not an OpenWRT issue, nor even restricted to consumer-grade routers. It is very common not to have an RTC of any significance in embedded devices. If you believe that you've got a situation where wireguard can't be brought up in a reasonable amount of time, then that is a protocol issue, not an OS issue.
Even with an RTC, you've got a 50-50 shot as to if it drifts forward or back. Some RTCs I've worked with have had drifts on the order of a minute per day.
Finally, one can easily implement a "safe" startup sequence, whether it be with scripts or the complexity of systemd. There is no chicken and egg problem.