If I leave the Xbox update preview right now will I have to go through a factory refresh?

What preview are you in?

Using the Insider hub >Manage should state what you're enrolled in.

Most of the time a factory refresh is required when leaving, changing a preview ring or leaving Insiders altogether. You will need the account details and passwords for all accounts that are set up on the console now to set the console back up after the factory refresh. Your games and apps shouldn't be affected unless you decide to do a complete factory refresh and delete everything. Xbox Insider FAQ

You can unenroll from the preview program through the Preview Dashboard app. I'm not sure what the current consequence of doing that right now is, you may have to factory reset in order to leave (was this way when the NXOE was being tested) - the alternative
that has happened in the past is your console will remain in the program until the current wave becomes publicly available, and then it won't receive the next wave of preview updates.

G'day,

Are you in the Insiders preview program?

If you are, OSU's (offline system updates) should NOT be used unless you first leave the program. Leaving the program usually entails doing a factory refresh which will install the latest General Public release.

"Done all trouble shooting, reset Xbox, updated firmware offline same problem"

If your console is enrolled in a Insiders preview OS you cannot use OSU1 (offline system update) to recover the console from E206 error.

Factory refresh the console keeping games and apps.

If you are NOT in a Insiders preview and want to attempt an offline system update, make sure you disconnect your wired ethernet connection before starting OSU1.

Personally I wouldn't bother with offline update and go straight to Factory refresh deleting everything if keeping games and apps fails to reset the console.

After factory refresh, reconnecting your console to your network should start a system update download to the latest OS available.

See also ....Whats new system updates in the Hardware section of Xbox support articles and /or search for E206 on that site.

G'day

I know you said without XBOX, but thought I'd advise how you take your console out of the Insiders from within the Insider Hub on the console or at least drop down to a lower ring.

After you load hub under Announcements you will see what ring (Beta) your in..

Click to manage and/or exit from the previews.

At of today you would have to factory refresh if you leave but could go down to Delta without a refresh.

If you go down a ring wait until the Omega ring OS version is the same and go down to there which has fewer updates and is nearly about to be released as a General release.

Hope this helps