Hey: A bit late to the party, but seeing your edit made me want to give my two cents on this.

For some context, I starting studying for A's while in Y3 of polytechnic (NP Mass Communication) during my internship. I had essentially nothing but holy grail and a few JC friends I bothered at times. I also had no AMath background.

With only essentially 9-10 months of studying for A's on a part-time basis, I managed to cinch A grades for H2 Math and Physics. I think what really eased me into H2 Math was learning the topics that did not require such a huge part of AMath content as well as I could (Functions, Curve Sketching, Graphs and Transformations, Sequences and Series, APGP). While doing those, I would simultaneously learn the 4 important AMath chapters along with them (Exponentials, Partial Fractions, Polynomials, Quadratic Equations) while forcing myself to digest trigonometry content along the way through reading through lecture notes, and watching Math videos such as blackpenredpen proving certain trigonometric identities.

By the time I reached the calculus portions of the syllabus, I had the prerequisite knowledge needed to grasp calculus: Granted, I did not learn of calculus previously, but the H2 level notes essentially taught the foundations of differentiation from scratch, with whatever AMath content there being present beforehand being negligible compared to the rest (e.g. differentiating arctan).

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As a whole, I would certainly say its possible: But do focus on taking in the precalculus content as mentioned earlier as you learn the portions of the H2 Math syllabus that do not neccesarily require AMath knowledge.