Every tank so far had some bonus material, I'm pretty sure the last one will too.
I think this ending was a little low key, but fit the series. Another 6 years on the timeskip so Yuzu was 18 would have felt like we missed too much, this let us see Mimika keeping to her promise and show that they still cared.
We know adult Yuzu is still cute, we saw her in the first volumes bonus material; non cannon it may be but no reason to think the design would change!
I'd love to read more, but I think the series was having trouble seeing where to go without forced drama or straying in to areas it seems pretty clear the author didn't want to hit.
All in all, a really lovely series I'm very happy to have read. I hope Ejima Eri does some more yuri works!