Suppose you had just specced out the home in it's entirety and turned the job over to a contractor, how much more would it have cost you in percentage terms? Would there possibly been fewer delays in that case (you had mentioned unavailability of certain tradesmen at critical times)? Would a contractor have had to commit to a completion date with related penalties, thus making your move-in date more predictable?
Most general contractors build custom homes at cost plus 12% to 15% management fee. So $500,000 construction cost = $60,000 to $75,000 for the management component.
Huge developers (Bosa, Chard, Concert) rarely deliver buildings without a 1 to 3 months delay so with a single family general contractor you probably wouldn't get too many guarantees either. A lot of things are also beyond the contractors control such as the client taking their sweet time to pick out flooring, light fixtures, etc., which may put things behind schedule.
It's not just the unavailability of trades (I use that as a loose general term). It is a lot more complicated than that because price and quality need to be factored into the equation. If you are willing to pay top dollar for sub-par quality than things would probably move forward without delays.
For example, let's say you want to frame a 4,500 sq/ft house and you call three framing contractors.
Quote #1: $27,000+GST, says he has a crew of three carpenters, no recent reference as he just came back from Alberta, can start next week.
Quote #2: $33,500+GST, says he has a crew of four, you visit a current job site and the work is okay, everything is reasonably square/straight but not perfect. Has two more small houses to frame, can start in 4 to 6 weeks but can't give specific date.
Quote #3: $44,000+GST, huge crew, currently finishing up custom home in Uplands, work done to perfection. Can start in three weeks.
Which one do you go with?
Framing is relatively easy because you aren't carry much cost....but let's say you are towards the end of a project, paying rent somewhere, carrying a $600,000 mortgage, for example, and the home still isn't finished. Makes it even tougher balancing quotes/timing/etc.