My concern with the home equity loan strategy is I'd still have the first home mortgage, the equity loan, and a new mortgage (albeit reduced from the down payment).

Meaning any of your Helocs can be closed or reduced at any point if any of your banks feel you are over leveraged, miss a payment, credit score drops, even by mistake it happens all the time.I would get rid of the Helocs and put some cash in your bank so you can use that versus a Heloc. 

Their inflated costs and reduced services make them an unattractive option.

the purchase price would be the stepped-up cost of the asset. with this, she would only pay tax on the interest earned, correct? 

@Bao Vu The purchase price and related expenses from 2023 should be included in the cost basis for the flip sold in 2024, reducing your taxable gain.

do you guys prefer to only invest in newer homes  i.e. built after a certain year to reduce the possibility of having to do a lot of reno on wiring, plumbing, etc. or does it just depend on seeing the house and determining the condition that its in?

For me I invest my SDIRA money in debt investments which typically are ordinary income when using w2 income and use my hard earned cash to buy properties. 

If you work a day job in addition to your plans to pursue real estate in this way, then focusing in 1 city/MSA will drastically reduce your time spent focused outside your job, allowing you to keep your income high, your job safe, and your problems simpler.

This can help you estimate potential earnings.2.

I think your income easily justifies a common-sense, time-proven method: increase earnings, reduce expenses, save, and invest.