Time for Making a Prozbul

When is a Prozbul made: at the beginning of the Shemitah year, at the end, or both?

According to Halacha (Choshen Mishpat, Siman 67, Se'if 30), following the opinion of most Rishonim, the Shemitah year cancels debts at its end, and there is no need to make a Prozbul until the end of the Shemitah year. However, the Acharonim (Tumim, Siman 67, Se'if Katan 26; Shulchan Aruch HaRav, Laws of Lending, Siman 36) write that it is good to consider the opinion of the Rosh (Gittin, Chapter 4, Se'if 20), who holds that the prohibition of collection begins at the start of the Shemitah year, but during the Shemitah year, if the borrower repays the debt on their own, the lender is allowed to accept it and does not need to say "I cancel." [And the wording of the Shulchan Aruch HaRav there is that "initially, one should make a Prozbul on the eve of Rosh Hashanah," and the wording of the Tumim there is that "one should be stringent to make a Prozbul, and a God-fearing person will fulfill all opinions"]. Another reason it is good to make a Prozbul before Rosh Hashanah of the Shemitah year is that some of the Rishonim hold that the Shemitah year is one year earlier than the time accepted according to the modern calendar, and although this is not the Halacha, regarding the cancellation of debts, which is a monetary matter, if the borrower claims so, it might not be possible to extract the money from them.