SA 550. Partial translation. I will add more as I go.
Short, I know, but I’m going to quality and certainty here within my means. Credit to Jan Nattier who guided me indirectly. She has no idea she helped me.
It is also very likely, IMO, that this is a late addition, possibly an "Earlier"BT, but not necessarily an EBT.
The phrase you’re translating as tathatā would seem to be ñāya; or at least, that’s the Pali word that is in that spot. I can’t locate a Sanskrit version of this passage right now. I agree that the Chinese term looks more like tathatā than ñāya; but the sense of both terms is not very different, so that even in English that might sometimes be translated with the same term. So either the Sanskrit text had tathatā, or the Chinese is translating ñāya this way; without more context I could not say which.