The Flat is always going to be the problem child so you make sure it is probe tender in the flat, especially the thickest and center most portion of the flat.
The Point has a lot more fat and will be done before the flat. If you only probe the Point you are likely going to be fooled. Plus temps in the point seem to go up higher well before the flat but you can't really mess up the point so again concentrate on the flat.
When putting temp probes to measure temp try and get it in the thickest center most portion of the Flat. Anywhere else is not going to give you the temp readings you are looking for.
Briskets are actually difficult to get very accurate temp probes placed in the flat so I put in 3 from different angles and guess what, one of the 3 usually gets the job done hahaha.
Use the temp as an indicator of WHEN to start probing for tenderness. Tenderness is the indicator of a brisket being done. When a brisket is tender ALL OVER (especially all over the flat) then the brisket is done.
Finally, you may want to trim away the thin end of the flat and repurpose it or smoke it with the brisket and just pull it earlier. That thin end will just burn up into crust on you before the rest of the thicker brisket is done. Here is a picture of what I mean:
Brisket is a challenge and you are going through a common progression of issues and learnings. Keep studying up and keep at it and you will get it down in no time :)