Most of the premixed “kit” seasonings do not specify to hold or not hold.
They give the mixing instructions and then the recommended Temps for processing. I may have to try with a small batch (my buddies won’t mind) and see if it works out.

When using premixed seasonings like “leggs old plantation” that have sodium erythorbate in the ingredients list, can I still add ECA for some tang?

So just to be clear, Legg’s Old Plantation is a seasoning normally used for making fresh sausage and does not come with cure and is not smoked. Fresh sausages normally stay uncooked/frozen until cooked and served.

The sodium erythorbate in Legg’s Old Plantation is mainly there as an anti-oxidant to keep the sausage looking and tasting better while refrigerated or in the freezer, and is not used as a cure accelerator, since fresh sausage normally does not have cure added.

Now if you wanted to add cure and smoke it, you could do that as kind of a free-style technique.

Regarding adding ECA to it for some tang, you could try it, but it might eventually be released in the meat before it gets cooked when using it in a fresh sausage at the same time you mix in the seasoning.

If you thought you wanted to add ECA, I would figure out a way to add it into bulk sausage right before cooking and eating it.