However, if I purchased at today’s rates (or cash out refinanced it at high LTV), it would still be negative cash flow.  

What are the best strategies to fund and execute several BRRRR projects simultaneously while waiting for refinancing to free up capital?

Can be a good strategy, but my understanding is BRRRR is generally dependent on refinancing and pulling back that cash invested to repeat the next one

You want to wait after the work is complete before you get an appraisal and do not do it until you are refinancing because most lenders will want it through the AMC and you will be out your money.

Your refinancing costs + new fha loan would be about the same as just using a 5% conventional loan for your next property.

One needs to simply stroke a check pay it off then figure out how to get their money back out of the deal by either selling or refinancing this is the ONLY way to protect the sellers credit.

Refinancing  immediately prior to a sale - yes that is a cause for some seasoning. 

They chose a second home loan for their beachside property but later realized that refinancing as an investment loan was more advantageous when they started generating consistent rental income.

Further, a paid off purchase does not preclude me from refinancing in the future.

As far as refinancing, you can either do a conventional loan where they use your debt to income / DTI to qualify or you can use a DSCR loan which structures the loan off of the rent versus income ratio.More info on DSCR loans: DSCR loans won't use your income to underwrite the loan.