AR750S and AR300M uses 18.06.0-rc1, while other device uses 18.06.1. Because 18.06.1 hadn’t been released since AR750S had launched, AR300M and AR750S use the same imagebuilder.
No, update the kernel has a lot of work to do, so we will update OpenWRT as little as possible, unless the OpenWRT has a large changed or critical security bug.