Why change the core system so much? There is no need.
Just use monsters across the board with lower CR.
12 instead of 14 AC? 10% higher chance to hit.
Saving throws of 4 instead of 6? Same as having DC 2 points higher.
If you want to have faster fights/more hits, just decrease difficulty of encounters aka CR and viola, same effect.
So why change a system that already works? DnD 5e RAW has solutions for your suggestions without the need to alter the rules. So why change it?
