High assembly variance, fluctuation and complex process sequences lead to a permanently high need for training in manual production - combined with an equally high commitment of resources when the experienced colleagues support the new ones with advice and assistance. That is commendable but - hand on heart - also not very economical.
How about giving the new colleagues a full-time teacher who knows everything that's important? A colleague who is always available and points out deviations in good time and thus easily minimises errors. A buddy you can rely on in any production situation. A real mate.