HELOC vs Refi - Do I have a poor strategy?

House is paid off but we are planning a big remodel. Rather than a construction loan I think our best strategy is a HELOC.

We will need about $150k, and I am trying to decide if, after the house is finished, I should:

1. Refi to a 30 yr (rate under 4%)
2. Refi to a 15 yr (rate under 3%)
3. Go interest-only on the HELOC and invest what I would've roughly paid on a refi

I can get 1.5% during a 10 yr draw period on the HELOC. This would put me investing $800-$1000 a month in a low cost index fund. Assuming 7% interest, I can pay off the HELOC in about 8 yrs.

I know this is taking on more risk, but I would essentially be paying off the HELOC with the same amount monthly out of pocket as a 30 yr refi.

Is this too risky? I consider myself very disciplined, and we will have 6 months of expenses throughout.