$20k to $5-10m is incredible. That implies 250-500x valuation growth (for example you joined at series A with $20m valuation and the company is worth $10b), which means you hit a unicorn within the unicorns!
Was the company QSBS-eligible when you exercised? There are huge potential tax savings there.
Your shares were NSOs, correct? There are weird potential issues with 83bs and vesting ISOs.
It's worth thinking through how to factor your paper money into your investment portfolio. You could model it as $0 and have an otherwise standard asset allocation (e.g. whatever WealthFront recommends for your risk profile). Alternately you could include it as a huge illiquid stake in a US tech company -- as if your portfolio was 80% in GOOG -- and mitigate your overexposure in the other 20%. Or something in between.