Hey Sarah,
I've seen this plan many times. It's loosely based on a 'how to get to a million dollars in RE' book - though I can't recall the specific title.
Overall - I like this plan as a high level guide. It sounds like you have a lot more details behind the scenes, and while these details should not directly guide your decisions, they can guide your plan which will in turn guide your decisions.
Still, I do see some gaps:
Philosophically, I'm real uneasy with selling real estate during the portfolio growth phase. Real Estate is expensive to buy and sell, and when you're turning property every few years, you're paying a big premium to do so. I figure 5% of the house value to buy it/get it ready and 10% to sell it... those are big expenses that often don't make sense if you're holding a property for less than 5 years.
On a related note, I would at least not buy in year 3.
This plan is heavily reliant upon the market staying strong. If we hit another dip and prices drop, you lose your equity and can't move up.
It looks like you're assuming forced or natural appreciation of your duplexes. That's an aggressive assumption.
Be wary of taxes when you sell! In addition to the cap gains, I think the gross profits affect your gross income on your taxes - in turn impacting AMT and other taxes.
Based on scale, those are some cheap duplexes with huge cash flow assumptions. Not to be rude, but you may want to check your numbers on those. I'm happy to be a second pair of eyes if you like.
Based on scale, those look like some old duplexes. As such, expect to be hit with large expenses while holding them (just part of the game, and maybe you built that in).
I would change the gap between apt #1 and apt #2 to match the loan terms on apt #1. If you can get a 5-year loan, you should keep apt #1 for close to 5 years.
The plan is non-diversified. At three times, you are putting all of your eggs in one basket.
I don't mean to beat up the plan - just wanted to highlight some areas I see as relevant.
Happy hunting.