For smaller investment amounts, we often see investors providing private loans or notes to other investors for their projects, which can offer attractive returns with the right due diligence.

Conventional loan with a 20% down payment.

(Either with the roofing company, a heloc, or a personal loan.)Either way you will probably be way ahead on interest saved unless you have a sub 6% interest rate.

Sometimes you will get a seller who comes back and declares they want off of the loan, or a nasty letter from an attorney saying that you ripped off their client, or a letter from the State Attorney General that you get to visit them in their office.

Any good resources for learning about hard money loans?

Since agency loans don't allow you to put the loan in an LLC, ask your wife this. 

If a HELOC isn't an option due to Texas laws, you could consider a cash-out refinance with another property or a portfolio loan, even if you want to keep your current rate.

I think I'll have to wait 2-3 years to pay for the renovation unless I take out a loan for the shower renovation.

Income: Verified monthly income must be at least three times the rent (minimum $4,200 combined).

And you have to pay PMI for the life of the loan.