BTW, other than the DHCP lease renew 'spam' in the logs, is there a functional issue you're trying to deal with? OpenWrt's logs are in a ring-buffer, usually in RAM unless you've changed the log location. It is designed such that the log file doesn't grow beyond a certain size (when the file reaches that size, the oldest entries 'fall off the end' to make room for new entries). So there's no need to restart the OpenWrt device unless you're having other issues.
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There was another problem that I solved in the tmp folder (my other thread about DNS failure). That turned out to be increased log level on openvpn. I wrongly blamed DHCP renewal but it is not true.
Now, it is just the WAN DHCP lease renew 'spam' in the logs for now. I may miss some information in the log because DHCP renew entries taken so much space that everything else is flushed out regularly due to reboot. For e.g. this guy is trying to connect to my openvpn server.
Feb 6 13:07:50 2019 TLS Error: cannot locate HMAC in incoming packet from [AF_INET]185.200.118.57:57373
In addition I do not know if this frequent renewal will affect other functions in the router. For e.g. DDNS, vpn etc. I have started pushing the log to another linux box in the network with a real hard drive and lot more space. But it is just annoying seeing DHCP renewal every 600 seconds in the log.
I was planning to connect ISP lan port to openwrt WAN port but keep the same subnet. I do see now that my proposed solution is not going to work. I do use VOIP and I certainly want to avoid double NAT.
Regarding your logs, sure, I understand that it may be annoying. But it is easy to filter, especially if you're sending it to an external syslog server. You can even just filter it on the read operation rather than worrying about filtering the log as it is being written. If that's not good enough, you might be able to find ways to suppress the DHCP entries in the log file (I don't know how to do this myself, though -- not sure if this would be a simple task or a large effort).
Regarding your 2-router solution, yes, what you're talking about would mean double-NAT.