Traditionally I have sold the builder land and loaned him the money to buy the land and build the house with us splitting up the profits when it sold. 

We back into the "strike price", meaning the maximum price we're willing to pay for a property, by starting with a conservative ARV and then backing off our required profit, the rehab cost, the time value of money (carrying costs and interest), and a contingency reserve for unexpected expenses.

Unless you absolutely love flipping I have often seen it is a means to generate short-term capital that you can then put into rentals (aka long term wealth). 

If she pays, then your really out of luck and you have to sue her in court for breaking the lease and damages.In general, I don't take people to court once I have the property back as the attorney fees and the fact you RARELY collect on any judgements (15% of the time or less) eats any profit and wastes your time.

Fast forward three years: He’s moved out, and the property now nets him close to $1,200 a month in profit after expenses.

Income generated from within CA has to be reported in CA.

Like Kevin stated, they don't care about your business even though Im sure you are not only generating income for the area the cabin is in (generating local sales tax) but presumably you are paying taxes not only on your property but on the income you make from STR biz. 

Am I being dumb by NOT waiting, because it’s unlikely - I assume - my next deal will be as profitable (I got very lucky on this one!)

I worked them, remodeled them and made very little money and eventually sold them for little to no profit.

If there is no agreement signed and this person is just someone listed from the broker's database, I do not believe it is acceptable for the broker to collect a split in addition to the house split.If you are to consider negotiating this additional split, there is a lot to take into account here... what is the % of business that you generate on your own vs. how much your broker contributes?