>Uuuhm, 9 months is too long for a normal skill set usable elsewhere. This probably is something specific to your company or project. That should reflect in lower pay at start, higher pay later - with option to "earn" higher pay if dev is up to speed much faster then you as stakeholder expected.

Yes it will take that long.... Its a large product in an very extensive project. Its like an octopus. The main executable is about 40mb large. the developers have to grasp the domain knowledge, which is challenging in itself. Part of the problem is that we are so distributed and therefore a lot of developers work remotely (especially in this pandemic) so communication is critical.

On top of that, we are growing rapidly with all the growing pains. The people who have to help the new ones to get started are overstretched and overloaded.
That is not including all the trouble from the organisational stuff.