You're probably right, Ben
The pattern, so far, seems to be TGIDs in the 35xxx range are Dallas County.
Various Dallas Services, such as DWU, the Convention Center, as well as some streets & sanitation, are in the 33xxx range, with some specific TGIDs for Dallas City Security in the 326xx range.
Dallas (city) events, so far, are in the 30501 to 30516 range. Just a wild guess, but I would not be surprised to see State Fair of Texas talkgroups in this range (305xx?).
Once we start hearing PD & Fire talkgroups, I would expect them to also stake out a specific range of TGIDs.
The other unknowns are where will the smaller cities end up. Cockrell Hill is currently using 6200 ->6202 on the Dallas Simulcast site of the Dallas P25/NCTCOG system. Other cities yet to be heard from include Hutchins & Wilmer, along with the 'Best Southwest' cities (Cedar Hill, Desoto, Duncanville, & Lancaster) which will transition from their existing NXDN systems.