I have good personal credit (~730-740), average-high personal debt (mortgage, student loans, vehicles,) and I am employed making $90k/year.

However, with mortgage rates being so high when I bought, I am worried I will have some negative cash flow (approximately 200-300 per month as a long term).

A heloc is an open end mortgage or in simple terms its a high risk credit line similar to a credit card that sits in 2nd lien position adding another trade line to your credit and reducing DTI.Just like a credit card if you ever miss a payment on any debts on credit or if your scores drop unexpectedly they bank can close or reduce your line of credit.

Unfortunately you are going to get different answers from different people, especially from Realtors trying to solicit business in their market.What I have been noticing is that even in landlord friendly markets, prices are high.

The rate will be high though. 

sorry to say it but there is nothing dishonest to engage in a conversation with an Agent that has a house on market since 200 days and offer to buy at a lower offer price - seller they believe owns the Taj Mahal and expecially after pandemic they raise asking price way too high - once again, guys we are entertaining a conversation with a seller and if the house is on market since 200 days, guess what, I believe the agent and the seller maybe a bit motivated, or what? 

:Class A Properties:Cashflow vs Appreciation: Typically, 3-5 years for positive cashflow, but you get highest relative rent & value appreciation.Vacancy Est: Historically 10%, 5% the more recent norm.Tenant Pool: Majority will have FICO scores of 680+, zero evictions in last 7 years.Class B Properties:Cashflow vs Appreciation: Typically, decent amount of relative rent & value appreciation.Vacancy Est: Historically 10%, 5% should be applied only if proper research done to support.Tenant Pool: Majority will have FICO scores of 620-680, some blemishes, but should have no evictions in last 5 yearsClass C Properties:Cashflow vs Appreciation: Typically, high cashflow and at the lower end of relative rent & value appreciation.

Any insights and recommendations will be highly appreciated.

I'm looking to invest the extra money into high cash flowing assets to take some pressure off.

Will check out that book as a high level overview