One of the things you can start doing is reviewing your credit score and work on improving it if needed.

Credit agencies ding you for having a statement balance.

We bought it for $225k with a 3.125% mortgage and 1) are interested in keeping it as our first rental. 2) We are thinking about using some of the equity (maybe $50k or less as a HELOC to combine with existing cash for our next home down payment that we can convert to a HELOAN through our credit union).

Second, try to ask for "LINES OF CREDIT" instead of HELOCs.

Most of my clients run their SFRs at a 25%-30% (excluding vacancy/credit loss) operating expense margin and most of the banks I deal with on their DSCR loans underwrite using 35%.

Quote from @Jeff Chisum: If a DSCR originator (keep in mind you don’t need a mortgage license to originate) tells you the loan will not be reported on your personal credit but require you to be a personal guarantor your next request should be; “I want to see in writing it will not be reported on my personal credit”.  

Now, some of these answers will depend on what state that property is in, how much equity you have, and a couple of other factors...but you can certainly seek out other lenders about a Line of Credit on an investment property.  

That works much better at 50+ unit properties.I have made the decision to invest exclusively in SF homes a long time ago, because it requires the lowest amount of management and provides the highest revenue per unit.

I called Home Depot, they told me to dispute the credit card charges so I did.  

sometimes to reduce borrowing costs, I'll use a hard money loan solely for the purchase, and then pay for the rehab in cash or use a 0% credit card (I don't recommend that for new investors!)