For the hiragana mono legends, please consider:
- Including an R4 「ろ」 key (possibly replacing the R4 「む」 if you have one)
- Including an R3 「む」 key (you may already have this for ISO support)
- Including an R1 「ー」 key (possibly replacing the R1 「む」 if you have one)
- Including a 3U Space key
- Including a 2U Shift key
In lowering order of importance. This set of keys is all JIS users need on top of ISO to be able to use the keyboard to actually type using kana. The first two are the most important, and relatively easy to add if you already support ISO. The third also makes a big difference, otherwise we have to take the "-" from the number pad which works but looks a bit weird. The 3U space is required to make space for the 英数/かな keys either side of the space bar (these can be bought individually from AliExpress, but so far I've only found them in black or white so they won't match). And the 2U Shift makes for a nicer-looking layout but we can get by with two 1U keys if needs be.
I know you have a lot of considerations to balance so I will understand if you can't manage all of these requests, but I wanted to make you aware of them in case there are any you can incorporate relatively easily.