No, the most important aspect is not flying the aircraft from the RHS. Whilst an accurate demonstration is, naturally, desirable, what is fundamentally more important is comprehension on both sides of all aims and objectives. That embraces what to look for, listen for, what to say to and when throughout any lesson - long briefing, short or preflight, the flight itself AND how to critique/debrief effectively.
Your ascertation that there is “3-4” hours of new material is flawed. Last time I checked the 25hours of explicit “Teaching and Learning” or the core instructor course didn’t appear in the ATPL theory.
By basic mathematics the 125 ground + 30 hours flying should take 155 hours which is just under 4 weeks using a standard 37.5 hour working weeks and then add breaks, lunches and down time R&R etc. Time sure flies, doesn’t it?