Yeah I’m not quite sure how it is done in real life as normally on an ILS you’d want 180kts at more than 8 miles out, 160kts between 8 and 4 miles out and then final approach speed at less than 4 miles out but RNAV doesn’t really allow for that. Or at least not in the A300/A310. In the A320 RNAV works the same as ILS in that you just press the approach button and don’t think it matters if not fully configured
Could be the order you are doing it. I always think level hold, final 3.0, profile and it works almost every time. Had a couple where I’ve had interpreted the chart wrongly and had to descend further before trying again but it always gets there eventually. Since getting the A300, I’ve been doing quite a few cargo ops routes in the US which involves a lot of airports with RNAV approaches
You probably could disengage auto throttle and it might still work manually. I’ve never tried it as I normally keep auto throttle on until around 400ft to avoid chasing around trying to find where the throttle was set which can very quickly destabilise the approach