Here's my stupid question:

I have 153k$ left on a 189k$ mortgage. My APR sits at 2.89% (got it in 2019), I was told by bank clerk that it might be worth it to call back early this year to get a lower rate; haven't gotten around to doing it just yet. With said mortgage came a HELOC on which I have 36k$. I've read online that these are often used for renovations to increase the home's worth. I bought a new condo so there's nothing for me to renovate.

So I've been wondering, how unconscionable would it be to use the HELOC to invest? I have no other debts (no credit card debt, no auto loans, etc.). I work as a UX designer and make a decent living (86k+bonus). I have no children or any other forms of responsibilities. To make the proposition sound a bit safer, I would plan on paying it back gradually with each paycheck.

I assume that as long as I can make more than 2.89% in returns from my investments, there isn't too much risk, especially if I pay it all back as fast as I can. I would attempt taking out 10k, and investing that for year to test the waters. I'm mainly thinking about optimizing opportunity costs/time in market.

What is wrong with this?