Your second message was just a little bit changed to your first.
Most routers are fully supported in OpenWRT “vanilla”, that is from the OpenWRT site, without the GL interface.
If your product is no longer supported by GL in the future, you can use OpenWRT vanilla, until they also end support for it. From then on, you would have to merge upstream security patches yourself.
As Kyson wrote you also, all GL v3 firmwares at the moment are based on OpenWRT 18.06.1, which is the latest version available.