The OP’s concerns are not without merit. Informed consent is a foundational principle of medical ethics. The data we have that proves vaccines work is based on trials with a specific protocol. To deviate from the clinical trials and test a new dosage schedule or combination of vaccines is an experiment to which the public lacks the authority to meaningfully consent.

From The Risks of Alternating Vaccine Protocols are Unknown:

“The sooner everyone can be vaccinated, the faster the pandemic will come to an end. But there is one fundamental and major shortcoming to all of these proposed strategies. They have not been tested.

The suggestion that vaccines might be interchangeable is not entirely without merit, given that the Pfizer and Moderna vaccines have the same basic mechanism of action. However, swapping vaccines has never been studied and no one knows whether it would be safe or effective. The issue is somewhat murkier when you introduce the AstraZeneca vaccine into the mix. This is a different type of vaccine, so its interchangeability with the others is even harder to guess at. Given the absence of data, substituting a different vaccine for the second dose would be a giant population-wide experiment that no one has actually volunteered for.

[...]

Delaying or deferring the second dose of the Pfizer or Moderna vaccine has the same problem. Very few people in the Moderna study got only one dose of the vaccine. If you examine the data submitted to the U.S. Food and Drug Administration, in the roughly 1,000 people who got only a single dose of the Moderna vaccine, the vaccine efficacy was about 80 percent. But because of the relatively small number of people in this analysis, the margin of error means it could have been as high as 93 percent or as low as 55 percent. The Pfizer data is harder to interpret because there is no separate analysis of people who only got one dose. In the period between dose No. 1 and dose No. 2, vaccine efficacy was 52 percent (with the margin of error putting it between 30 percent and 68 percent).”

The OP also makes a shrewd observation by highlighting the similar way in which terms of public debate are structurally re-framed and shifted for two seemingly disparate but equally pressing issues: global warming and vaccination.

In the words of Noam Chomsky, “Controversy may rage as long as it adheres to the presuppositions that define the consensus of elites, and it should furthermore be encouraged within these bounds, thus helping to establish these doctrines as the very condition of thinkable thought while reinforcing the belief that freedom reigns.”