I don't see a big difference in the time taken from saying "Ready
for takeoff" to saying "Holding for IFR release".

Making something simple seem obscure. Clearance Delivery gave me my
clearance; all I need from tower is a clearance to takeoff. Tower
needs an "IFR Release" for me, but that's their problem, i.e.,
"Controller Stuff".

If saying that I was holding for an IFR Release had some value, seems
that the AIM would suggest the phraseology.