Flax and chia are not a sufficient omega-3 source. Specifically, you need omega-3 EPA and DHA supplements. Flax, chia, canola oil and most other vegetable-based sources don't have much in the way of omega-3s usable by the body--it’s all ALA, which converts very inefficiently to EPA and DHA.
The Amazon algae oil supplement you linked is fine; algae oil is basically the only (afaik) vegan source of EPA + DHA.
This is sort of true in the sense that seitan is almost pure protein, but as proteins go, seitan is a poor source of lysine because it only has 57% of the recommended amount of lysine per gram of protein (source: https://doi.org/10.1002/fsn3.1809).
Flax and chia are not a sufficient omega-3 source. Specifically, you need omega-3 EPA and DHA supplements. Flax, chia, canola oil and most other vegetable-based sources don't have much in the way of omega-3s usable by the body--it’s all ALA, which converts very inefficiently to EPA and DHA.
The Amazon algae oil supplement you linked is fine; algae oil is basically the only (afaik) vegan source of EPA + DHA.
Minor note:
> Lysine-rich foods include [...] seitan
This is sort of true in the sense that seitan is almost pure protein, but as proteins go, seitan is a poor source of lysine because it only has 57% of the recommended amount of lysine per gram of protein (source: https://doi.org/10.1002/fsn3.1809).