The main reason I'm asking this, is to understand how much cash I need to save up in reserves before pulling the trigger on a deal like this?

It’s an investment that will likely pay off in terms of durability, maintenance savings, and overall property value over time.

We still own this property, but we have: 1) leased to market rent, 2) successfully implemented RUBS to save roughly $3,000/year, 3) Reduced vacancy/bad debt from pre-acquisition T-12 of 15% to less than 4%.

If steady cash flow is a priority, option 3 adds resilience to your retirement.

Use an operating account for income and expenses, a reserve account for maintenance and taxes, and a high-yield savings account (HYSA) like Capital One's 4% APY for idle funds awaiting reinvestment.

There is short-term savings in replacing my mortgage with my credit line.

Use your savings and equity to get the next one and keep this one. 100% agreed here. 

I kept quite a few, but would be retired had I kept them all. :(  

A good HI will go over your house with a fine-toothed comb.If you can find an Ex (or Retired) HI that is also Certified, that would be even better, since they will understand the structure/electrical/plumbing systems in more detail than a regular HI...

How much time are you saving by doing things more digitally and with more automation?