some of my tenants do not use electronic because of limits the companies like cash app use on their system.

RE investment can help you do that if selected right property but I would invest just for that reason because it always might not work and there are limitations.  1) Depending on your retirement goals, you will be better off with a qualified plan(profit sharing plan) for your S-corp that will shelter more of your income from tax and also lower your AGI.

Assuming that the conditions above are satisfied, wouldn't it be the best option to search for a deal that is as close as possible to the maximum FHA limit, and the one that has 4 units?

Also they may limit your ability to obtain better rate mortgage as you may only have commercial rates available.

Does anyone know any other lender who do not have such limitation on non owner occupied HELOC loans?

You will be limited on your options but a small lender or HML should have an option and then you can refinance out. 

However if your time is limited or you have absolutely no interest to be active in real estate for whatever reason, then investing via syndications is a great way to invest considering its benefits.

You will also get the best rate and limit the fees you have to pay.

They charged 5k per person to increase one’s credit limits and help them access business credit lines. 

Once you get bigger, and exceed some of the limits of conventional mortgages (4-6-8 properties) then you’ll be in a better position to start looking at commercial loans and make the switch over to an LLC.