The chromosomes copy at S phase. So S/G2 checkpoint up to early anaphase has 2n. You are on the right track to understanding the cell cycle, important to note the differences between homologous chromosomes (homologous pair) and sister chromatids, while understanding ploidy.
During S (synthesis) phase which occurs between G1 and G2 , all the somatic DNA replicates. So every chromosome gets copied and thus has a sister chromatid.
Animals that undergo sexual reproduction have 2 sets of chromosomes, one from mom the other from dad. The chromosomes of each pair represents homologous chromosomes.
Your answer is almost correct. I believe it could be adjusted to:
G1: 6 chromosomes
G2: 6 chromosomes /12 chromatids Prophase: 12 chromatids
Metaphase: 12 chromatids
Anaphase: 12 separating chromatids
Telophase: 12 chromosomes, 6 one each pole of the cell
These very simple figures explain the principles I mentioned earlier