Based on the data available to date I see no basis to support your hypothesis.
Its not a hypothesis, it is a fact the majority of the Australian population will get two doses of an MRNA vaccine (predominantly Pfizer, a few Moderna).
Second doses of Pfizer have been given out every week since middle of February and will continue until the end of the year, if not beyond - all those not currently eligible for a first dose are earmarked for an MRNA vaccine. There is no expectation that we will see a reduction in second Pfizer doses, only a continued increase as supply increases.
Second doses of AZ only started in June and are likely to end well before the end of the year, as over 50s have been able to get their first dose for several weeks already, there is no shortage, so anyone over 50 planning to get vaccinated will likely have had their first short by end of July, so we should see second doses of AZ wrap up by October (most before then).
Yes we will see a increase in ratio of AZ to Pfizer in June-August period, but it is temporary and the overall doses of AZ will NOT eclipse the total number of Pfizer doses given by the end of the year which was my point.
there also under 50's who were given AZ prior to 9th April, and others under 50 who chose to have AZ after 9th April who are also still awaiting their second doses
However, there are also all the over 50s in residential aged care who got Pfizer, and plenty of workers in 1a (and some in 1b) who work in HQ/Hospitals/Airport who are over 50 who also got Pfizer. Plus some of the under 50s who got AZ before 9th April are choosing not to get the second dose and switching to Pfizer.
The number of under 50s who chose AZ is less than the number of over 50s who were given Pfizer.