If your intent is to use that cash to obtain more cash-producing assets, then in theory you should be reducing your exposure.

More time, more bodies, more everything on a property opens more possibilities for risk/exposure.

Assuming you keep a decent buffer in cash equivalents your plan of slowly buying one rental unit a year with 20% - 30% of your own personal capital down won't bring you anywhere near the levels where I'd start worrying about excessive leverage.

The advantage to a line of credit is flexibility, but the trade off is exposure to a rising prime rate.

Why would I put excess share of my precious capital in a mortgage to save 4% APR, when I could buy another rental and earn way more?

The excess cash flow will be over $10,000 per month.Thanks,

Insurance seems a bit low but your in a different area then my limited exposure.

If I invest in real estate, I may be able to get returns in excess of 10-12%. if I invest in stock market I may be able to average 8%.

What is my exposure if I do go forward? 

When all of those constructions will be completed (depending on which asset class), there will be excess supply in the market, throwing off the balance of supply and demand.