What is the Difference Between The First 2 Covid Injections and the Booster(s)?

I cannot find any discernable difference.

If there is no difference would you have taken any injections if you knew you had to take up to 4/5 in a year and a half?

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OldBrit · 61-69, M

There is no difference ie same vaccine same dose (there may be some different for young people ie under 18s but ignoring that)
Some countries (like the UK) decided to use mRNA vaccines for the boosters only as there was some evidence that heterogeneous vaccine use provided a slightly boosted immune response over time.

So in the UK I got two first shots of the AZ Oxford vaccine (classic old vaccine design ie a modified chimp cold virus with the covid-19 spike protein on it) and my third booster was Pfizer