The least expensive bid isn't necessarily the best. 

Going to also guess that you've probably paid more to your contractor then work they've performed.

There are fees to be paid with rent control.

But even if you only wanted to pay your mortgage off faster and not invest, it works as long as you are disciplined, don't increase your expenses w/o at least a corresponding increase in income and basically use the HELOC as your checking account.

Would there be rental expenses I could write off that would more than compensate for the marginal income?

It prevents me from transferring money to my crypto exchanges and I end up wiring money, which gets expensive.

After all expenses it cash flows about $450/mo.

It’ll be paid off in 10 years.

Borrowed 250k from a private money lender (local attorney) at 10%, to be paid in a balloon upon the sale.

You will not be able to assume the loan, the first will eventually foreclose on the property unless it is paid off.