Logically, it can’t be kefira to say a rishon is incorrect about something. The rishonim disagreed among themselves anyway, so someone has to be wrong.
Neither it is kefira to say Chazal weren’t 100% correct about everything. If that were true, then the dozens of rishonim who held that Chazal erred in scientific matters were kofrim, c”v’s.
Similarly, it’s not true that every drash on a pasuk in the Gemara is necessarily authoritative. See these two sources:
“Rav Hai Gaon was asked what is the difference between those Agada and medrashim which are written in the Talmud and those that are not? He replied that whatever is found in the Talmud is
more valid that what is not found in the Talmud. Nevertheless, even those Agada and Medrash which are found in the Talmud if they make no sense or are erroneous are not to be relied upon. That is because in general we dont treat Agada as being authoritative. However that which is found in the Talmud, we should correct their errors if possible. That is because if they didn’t have validity they would not have been included in the Talmud. Those that we cannot figure out how to correct should be viewed like that which is not the Halacha. In contrast that which is not in the Talmud, we have no need to attempt to correct them and make sense out of them. We merely should examine them as to whether they are correct and nice. If they are, we teach them. If they are not, we pay no attention to them.” (Sefer HaEshkol)