If you use Insider on the Release Ring. The same version is fetched as the recent public Upgrade received by these phones (10.0.10586.164): Phones Eligible for Windows 10 Mobile Official.
If your phone is not on the supported list, then 10586.164 (Threshold Final) is the end of the line. You may as well use Insider and select Production to un-enroll your phone from Insider, but at least you have W10M. If your phone is on the eligible list and you go Insider and stay on the Release ring you get the next public version early as Release ring in effect is the RTM. If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Windows Central.