Hi,I'm a Security Engineer by day and looking to learn more about real estate investing.

It depends on his goals, but if he's looking for cash flow I would buy a secured first trust deed.

I’m a new landlord in CA and missed the 21 day window to return the security deposit (I thought I had 30 days).

Return on investment, with security of a safe neighborhood.

The more leveraged secured debt you have will impact any cash flow.As mentioned by Nicholas, the BRRRR method could help payoff the HELOC too. 

I would like to hear feedback/input from experts regarding any potential red flags.Could you share your insights on both short-term and long-term rental investments in Columbus?

Caught the flag on the first sentence "given the loan terms verbally" - Pretty much all the "reputable" DSCR Lenders will give out a detailed term sheet / pre-approval letter / LOI (many different names but same thing) outlining terms and approvals early on in the process - the good news is that if you haven't closed yet you can still switch lenders and see if you can secure your original terms - also good news is that the appraisal, while likely ordered by the lender or broker is in your name and your property, so you can likely transfer it to the new lender

You get an estoppel from each tenant saying this is how much rent I’m paying, this is how big a security deposit I gave, here’s who pays for utilities, when my lease ends, etc etc. 

@Sam McCormackCongratulations on becoming an investor and securing your first property in Northern Kentucky.

Getting pre-approved gives you a serious edge in securing deals faster.