I wanted to pop in to say that just because you can clearly distinguish some things in Forma does not make it a better choice. As far as I can tell, Kindle Oasis renders the images close to the original. Yes, at times, this is detrimental. You might lose some details because it is not as washed out as Forma.
But, for manga, this is indeed better. If only Oasis was larger, it will be my default device for everything. As of now, I still use it for manga when the manga layout does not make a larger device necessary. My previous images were for Jojo and I have been reading that on Oasis because the layout is fine for the size of Oasis and the text is clearly legible. When I was reading Vagabond, I completely relied on Forma because it made the dark inking clearer. However convenient e-ink might be and however better the contrast is getting, it is still nowhere near the actual contrast of paper. Voja had a video where he tested the background of several eink screens to paper
here. For manga reading, the choice is very subjective based on the inking and layout of the manga.
For text, as I already mentioned, Oasis is my preference not because of its better contrast but because of the size.
Let me close by re-iterating again that for reading text, it hardly matters which one you would choose. You can adjust the boldness and you will be fine most of the time. If you are concerned about reading manga or the front-light then that is a different issue.