Try to set the following:
System Preferences -> Dock -> Prefer tabs when opening document -> Always
Have multiple SublimeText windows open (with multiple tabs in them), and I relaunch the app. Now all windows are combined within one, meaning that I have two levels of tabs open! This is actually a really cool feature, if it was supported by SublimeText, but it isn't. This means that I have no way of recombining separate windows except for relaunching the app, cause all windows to recombine.
Adding support for this should be relatively easy since the system-tab support is obviously provided "for free" by macOS. All ST needs to add is support for managing these tabs. It's already possible to drag tabs out to separate windows, so we just also need to be able to drag them into each other (as is already possible with ordinary ST tabs). Also, add menu points for "next system tab" and "previous system tab", so both levels of tabs and be cycled with keyboard shortcuts.