Complex question. Try having no more than 3-4 project per PM if they are small.

1. Project management capacity is not a robust metric. Optimize project success not load efficiency.
2. Project velocity can be simply color coded. Green for nominal, orange for at risk, red in jeopardy.
3. All deadlines should be hard. If one is missed, diagnose problem, resolve root cause of problem, set new deadline.
4. Same with color code.

The secret is building teams that are empowered to make decisions, accountable and autonomous. It sounds tough, start with focus. Measure only what's important.

We can talk more in detail if you need more help.