Capital Gains Strategy for investment property for future primary

[Brad M.]

I have a lot to go through so I'll get going with the facts

1) I own two residences, 1 investment and 1 primary.

2) Investment:

• Purchase Date: 6/2013
• Purchased as primary, converted to rental 9/2016
• Purchase Price: 370K
• P&I, Est. Maintenace, Condo fee, taxes and Insurance: $2,030/month
• Rent: $2,750/Month
• Market Value: 590K
• Mortgage Balance: 300K

3) Primary:

• Purchase Date: 8/2016
• Purchase Price: 535K
• Market Value: 580K
• Mortgage Balance: 425K

4) Other:

• $175K in liquid assets

We live in a very expensive market here in Boston. We want to move to a suburb with great schools for our 15-month-old and the other on the way but don't need to move until they are ready to start school in 4-5 years. To get a livable home with enough space in towns like this, it will run us around an 800K purchase price. I don't have expensive taste, that's just what it costs around here. In towns like Brookline and Winchester 800K isn't even enough to buy anything.  According to the 2/5 year rule for the capital gain exclusion, we have until 8/2019 to sell the investment property without having to pay capital gains. After 8/2019 we would owe capital gains which if we owed this today it would be a tax bill of 63K. We have been trying to think of ways to come up with the funds without having to sell the investment and below are all the options I can think of with some questions if anyone can help...

1. Sell everything including the investment before 8/2019 and invest in liquid assets until we are ready to purchase
2. Use a 1031 exchange to be converted into a multi-family where we would live in one of the units and rent the other(s) if this is possible? 
3. Pay off the investment mortgage to increase cash flow in order for my family to afford rent while using the other assets/cash to invest in other properties. Financially this makes a lot of sense BUT the major problem is we don't want to move for a long time and being a renter doesn't guarantee anything. Anyone ever worked with a long-term lease before? 5 years for example?
4. Use a HELOC from the investment property to help with the DP

These are just a few options but I think you get the idea. We want the future primary home to ideally have a mortgage of less than 400K. Any ideas or insights would be greatly appreciated and please don't hesitate to ask any questions!!! Please keep in mind no option can have a sale contingency. In the Boston Metro market, you won't get an accepted offer with any contingencies.