I've almost given up asking for a clearance through Class D airspace
Seems, with some units, even one aircraft makes them too busy to issue a clearance - so I just plan to avoid - more time and more miles and therefore, more money in the planning but, these days, it's always a lot less than the actual reality of holding etc etc waiting for the aircraft on a 4-8 mile final to land before you get to move .... can't be too careful !
I do a lot of hours - this is the norm. in the UK.