I'm sure this is nitpicky to anyone that doesn't type in Japanese, but with the round 2 modifier sets it is pretty annoying that the Alt and 変換 and 無変換 keys are all the same. I would have to buy both the JP mod kit and the icon mod kit to get separate Alt and 変換 keys.
I've never had a keyboard that actually has 変換 keys, but I've been wanting to try to build one for a while now. I thought this would be the set for it since it has 3u spacebars, but an extra $50 just for two keys... And the fact that there's no ー key and the ろ key is in the wrong place, I wouldn't really be able to make a JIS board with it, but those are secondary to me because I use the Roman input method to type.
Short version - it actually isn't a JIS keyset, it's a ANSI/ISO keyset with almost all default mappings for 101/102 keyboards for IME printed on keys, so if you really try to build a JIS board you are hacking the design a bit.
Long version - I'm going to use Henkan and Muhenkan as the Alt keys as intended by design, and Fn-Alt can be the actual Henkan/Muhenkan functionality. Right now, spacebar activating your conversion popup is widely used and duplicates the functionality of Henkan (and is still a thumb key), and Muhenkan is pretty easily achieved with enter instead of converting (although no hirgana/katakana/half/full quick toggle there). So the actual dedicated keys are kind of a novelty, and putting them on FN I can live with. Thinking the same way you were, compared the 101/102 vs 106/108 keyboard settings with both types of boards connected, and explored each keypress and IME conversion functionality to see what might work and come to the above short version conclusion. One noteable in that area is Yen symbol on shift "\ | mu Yen" is not a default mapping in Windows 10 (and I'm not sure personally of an OS where it is), but I hope to work that out with QMK (and can be done in registry mappings if necessary), but QMK would make the board portable if I can do it there rather than changing multiple machine settings for me.
If you want to build an actual JIS board from it, you have larger problems than Henkan and Muhenkan, because the ke key, mu key, and likely others don't actually have a symbol match for the proper JIS keys. JIS has a ke, askterisk, colon key for example, and this kit doesn't have it since that isn't where they appear on other 101/102 key keyboards.