“If both Ashkenazi and Sefardi sidduring have something, it probably dates back at least to the period that the Talmuds were being written”
????? ????? Source? Please?
Today, most siddurim (particularly Ashkenazi siddurim) borrow plenty from Sephardi and Sephard siddurim. In addition plenty of the changes and additions that “survive” to this day are changes made by maskilim and/or due to censorship and/or kabbalistic additions/renditions which did not exist before the Ari, and in some cases not until the Shloh, etc etc.
The Seder HaYom is the accepted source for Modeh Ani according to both Sefardim and Ashkenazim as a “temporary replacement” for “Elokai Noshomo” which can’t be said until one washes his hands – and which tradiontally would not be said until one reached shul.
The Abudraham is seemingly the mekor for the part which says “sh’hechezarta bi nishmosi” – but the Modeh Ani portion is not found anywhere before the Seder HaYom to my knowledge.
“Physical sidduring don’t last very long since they are heavily used and wear out, whereas a nusach will survive and leave its trace on future siddurim.”
There are B”H a decent amount of siddurim that are still in understandable or printable condition from many years ago.