"where to find the highest cash flow city in USA ?"

You make extra cash (all taxable) but the longer the stay, the higher the cost.

Purchase price: $150,000 Cash invested: $150,000 Sale price: $400,000 this was my home for 15 years, was an old mobile home but I bought a new one July 2023 paid cash. 1460 sf. 3 bedrooms, 2 full bathrooms, beautiful granite kitchen, beautiful back lake view with a boat dock, adde a new front porch 10x12 and adding also a 22x18 back porch, private neighborhood, estimate of my home is around $400,000 What made you interested in investing in this type of deal?

Where is the renovation cash coming from?

Worse case, if you do the rent decrease, is that you have a non-cash flowing property/rentals below market rent as well as you have tenants who expect their rent to be lowered at every inconvience.

You will probably find there will be little cash leftover if you hire this out so I'd encourage you to try to manage it yourself to begin with then if it's not sustainable for you, try to find someone to take it on.

2) Who are the biggest cash buyers of properties?

Same with cash, other loans, and your time. 

Purchase price: $240,000 Cash invested: $30,000 I purchased this house to renovate and add value to!

Done correctly, you can work deals with neither risk nor upfront cash.  30 years ago lease option assignments was my start in this business and I still utilize this strategy today.