thank you again for taking the time to test. i was also confused at first so it was good to finally understand the system completely.
regarding figuring it out, actually a lot of these mechanics are copied from granblue. i know about the weapon bonus by type because it is important in granblue regarding long term weapon grid building and character selection, and then after confirming the weapon bonus exists in kamihime, i tried to see if there was similar factor for eidolon.
as mentioned on first page, even though simplified, kamihime is still very maths-heavy game and a lot of same strategy for progression is the same as granblue.