I think MLs might work well, especially since they have a tactile feel. Other than that, a microswitch with a small lever might be perfect to get a little more travel out of the microswitch.
4-7N is extremely high, roughly an order of magnitude more than a "normal" keyboard. You could get some custom springs made for a ML switch or something. If you can 3D print, a custom hall effect switch over custom springs could work as well. You might have to du custom leaf springs or beam springs or something to get that amount of force. I'll think on it.
To compare, a normal rubber dome keyboard is about 85-120 cN at actuation (all the way down), and a normal mechanical is about 50-80 cN at actuation (roughly halfway down.
---
To DIY scissor switches would be quite hard. You could do carbon contact switches over PCB without too much difficulty, but getting a custom membrane made will prove difficult.
Hope this helps and letus know of updates! I'm interested for sure! I made a custom gaming keypad and I love mine.