Most new engines idle at 2000+ rpm and run around 10,000 rpm
The magnetos are designed to provide the best strongest spark at full speed and just enough of a spark at cranking speeds to start.
Cranking is a lot faster than most people would believe
And no, without battery power you can not have our cake & eat it
either the spark will be good at full speed or good at idle can't do both without complicated electronics that in practice have all failed so been abandoned ,
I have pronounced brand new coils dead in the box only t find out latter they were just not getting enough RPM so now I use the bench drill set as close as possible to idle speeds