Well now,I haven't had the pleasure of cooking on an Elite yet,but I have probably had more failures than anyone on the forum.
I assume you have read smokin's Brisket 101.It gives you much of what to do and gives you enough good info to ask the right questions ,that we might be able to give the answers that are helpful.
Whatever we do,the quality of the starting product may dictate the quality of the end product.
Some compare the hunt to your going to the Alabama vs Notre Dame football game tonight,having a few beers,stopping the first person of the opposite sex and finding your spouse for life.
Suppose it happens,but it takes a few tries-usually.
Too stringy could mean it was cut with the grain,rather than across it.If cut across the grain,you could cut thinner slices.
I suppose the therms were checked for accuracy and the temp was checked in several places?
If so,internal temp means it could be a good time to start checking for tender.
If your temp probe passes thru from top to bottom like warm butter,it probably is ready to take out.You can also try to pick up with a large two tined meat fork and if it falls off,it could be tender.
If more questions,there are some experienced packer cookers will jump in.