Class4 and Class5 softswitch building with Kamailio / FusionPBX

We need a solution to replace our FreePBX setup. We currently have 4 instance of FreePBX, but this is not the perfect solution. Instead we would like two Class 4 softswitch for redundancy. in the front to give LCR and SBC fonction. We figured that Kamailio is one of or best solution and would then add Class5 switch, We would think that FusionPBX is a better solution than FreePBX. The setup need to be accessible with an API so we could provision customer without GUI. Maybe our software choice is not optimal we could then discuss our best solution. We only need to have it build once, we will then continue to operate and work on it by ourselves. Possibility to hire again if we eventually need help or want to upgrade our setup. Software should be able to communicate in French and English to everything facing customer.