If you cashout refinance your home and use a portion of that money, 10's of thousands, to purchase stock investments, can you deduct the interest as an investment expense

You should read the following TurboTax article.

https://turbotax.intuit.com/tax-tips/investments-and-taxes/what-are-deductible-investment-interest-expenses/L9TeFQAf9

Note that you can deduct your investment interest expense during the current tax year, but only to the extent you have investment income, such as interest, dividends, gains, et al. To the extent your investment interest exceeds your investment income, a carry over to the following tax year is created.

note that on the refi, only the principal balance at the time of the refi is considered mortgage interest. the excess must be traced to its use.