The Gemara in Yoma (28b), tells us that Avraham Aveinu observed the entire Torah, including Mitzvas like 'eiruv tavshilin' which are miderabanan.

If so, why did he wait to circumcise himself until Hashem commanded him to "Circumcise …all your males"?

The Gemara in Avoda Zara says that if a non-Jew circumcised a Jewish child, the circumcision is invalid (even if the non-Jew himself is circumcised) and the operation is considered a medical one, not a religious one.

Until Avraham Aveinu had been commanded by Hashem to circumcise himself and his household, he wasn't able to observe the mitzvah because in for all intents and purposes he was considered a ben-Noach (gentile), and therefore had to wait for an explicit order from Hashem.