I now recall why I did not buy the 6300 4G, because I remember it looked very promising: it is out of stock everywhere. I'm going to assume HMD aren't making more of those, and they don't seem to have any KaiOS feature-phones in the works.
I found mine on Amazon shipped from France. The reviews were all over the place with some people complaining that they had ordered a dual-sim version but only received a single-sim one, I took the chance and got the dual-sim one that I wanted, maybe some inventory co-mingling had been happening.
It is a shame that it isn't widely available, someone could maybe write a Signal app for KaiOS.