What is the best way to avoid as mush tax as possible?

It sounds like what you are describing is a flipping business, which is a business not an investment.Therefore, if your intent is to flip, then this is ordinary income and subject to self employment tax as well as income tax.Your profits for this type of business are Selling Price - (Purchase Price + Rehab Costs + Holding Costs) = Profit.If I'm reading your question incorrectly and you are talking about a property you have held as a rental for a while and you are then fixing it up and selling it, then this is capital gain tax.  

I'm trying to improve my methods for estimating expenses when analyzing deals, and I had a few questions related to property tax.For example, when I look at a property on Zillow, I might see figures like this:Why does Zillow show the property tax as the same each of the past 3 years, if the assessment value has risen?

Primary residences usually sell better and rent worse.And ultimately you are going to want to transition to passive ownership with as much captured deferred tax as possible while freeing up as much tax free money.I personally would not give up the entirely tax free primary until I was ready to leave the area or down size (given that you're close to retirement).  

The net earnings are only subject to income tax, as opposed to LLC, which is subject to income tax, social security tax and Medicare tax.

But you'll pay tax on the difference until  you've paid as much tax as you would have if not doing the 1031.  

Real estate lets you use most of what you have with little taxes (thanks to depreciation) or tax-free (thanks to refinancing).  1031 exchanges let you upgrade to larger properties without paying tax as well (in fairness, it's deferred, but indefinitely potentially).That's why I invest in real estate because I believe it's far superior to stocks (just my opinion).  

Some people will argue that the Solo gives you checkbook control, but you can set that up with a Roth/LLC structure too.Truly self-directed Solo 401k plan has built-in Roth, allowing to have both: pre-tax as well as post-tax (Roth) accounts within the same plan.

Or when a state like CA enacts a "gas tax" as they did recently .

There may be a state gift tax as well.