I bought my 2.1 tetrataenia from three different breeders in the Netherlands. Result was that this trio produced around 85 healthy young of which the most are still doing well as far as I know. They were all from the same male.
One daughter I kept and placed her together with the other "old" male. She gave a first litter of 10 that all do well.
In this way I seem to have a good breeding line.
What will happen when I add one or two specimen from another line to the breeding group? In theory this should be an improvement. But since all European tetrataenia are related in one way or another, you will never know if it will be an improvement. No one will know this in front.
I have the idea that there are a number of breeding lines in Europe that do well and a number of other lines that produce poor quality juveniles. Most of the breeders with the last mentioned lines stop breeding tetrataenia.
In this way only the fittest will survive. Next to that, there is no choice. There are no specimen available that are not related and most probably there will never will be.
I now have some young that are the future breeders. I hope to exchange one or two specimen with other breeders and than wait and see if this "new line" produces healthy young or not.

Normally I like to "inbreed" as little as possible, but with tetrataenia this is not a realistic option. The only thing you can try to avoid is that primary related specimen (father-daughter, mother-son, brother-sister) are used to breed.
But on the other hand... maybe healthy, perfectly functioning animals that are primary related will produce better young than when you add other "related" specimen to your breeding group (nephew, niece, grandniece, etc...).
You will know for sure after a number of years.

Very important is, when you think one of the snakes is, to your opinion, not suited for breeding, you end its life or keep it yourself and do not breed with it.