Suppose, I‘m getting $12, 000/year ($2k/month) after deducting all taxes and expenses.

They are learning bout limits, deductions, liability, etc.They meet with my attorney for my will and Trust, and know how they are set up and who will eventually manage what.They know that business is business and charity is charity, and are learning how to apply that. 

First, you must meet the eligibility requirements to set up a Solo 401k: (i) you are self-employed; and (ii) you do not have any full-time w-2 employees (i.e. working 1000 hours or more per year) working for your self-employed business or otherwise working for you.

I don’t think any landlord who is trying to make a profit does You know what a real landlord would do...Cite the lease, fix the problem, and if you they don't pay deduct it from their security deposit.I don't think any landlord trying to make a profit wastes their time getting in texting battles with tenants over chipped paint, pal.

There is even a school of thought to use market rents then deduct 10% vacancy, then 50% expenses and that should give you NOI (rule of thumb).

The principal portion of your mtg payment is not deductible.  

Is the cost of the windows tax deductible?

While you certainly can (subject to the contribution limits), your ability to make deductible contributions to an IRA is limited if your AGI is too high.

First, in order to set up a Solo 401k you must met the eligibility requirements:  (i) you are self-employed; and (ii) you do not have any full-time w-2 employees (i.e. working 1000 hours or more per year) working for your self-employed business or otherwise working for you.

This also now comes into play with the QBI deduction since there's a limitation related to qualifying assets  well.