So my RJ9 connectors have 4 stranded wires of pretty small guage, smaller than 24AWG I think. I tried to crimp DuPont connectors directly onto those and I was able to briefly get connectivity and both halves of the keyboard working together, but the actual connections don't have enough friction and so just fall off with the slightest movement. So it's no good. Next idea will be to use store bought DuPont innie connectors and splice those wires to the RJ9 connector wire. This is basically what I did in the previous build (which was done before I had my own crimping tool). So it'll be a bit fiddly to do all that splicing with the connector already glued into the slot box, but it should be doable.
I'm a little nervous about gluing the PCB mounts into the slot box as once I do that, futzing with any DuPont connectors will be really tight clearance and a nuisance. Hopefully I don't need to undo the bolts so the PCB comes loose in order to sure up the connectors.