Which Arduino board is best for beginners?

Getting started with Arduino can feel quite overwhelming at first because you're faced with the decision of what board to buy ...
GitHub

megaAVR_Slow_PWM Library

As more complex calculation and check inside ISR are introduced from v1.2.0, there is possibly some crash depending on use-case. You can modify to use larger HW_TIMER_INTERVAL_US, (from current 33.3uS ...