When should I form this partnship and open a business bank account to pool capital?

You could just hire him to build; the PM piece shouldn't be tied to the JV - they get the business if they do a good job and it should be equitable fee's for the area. 

This also translate into being a real estate investor, into business, studies; you name it.If I'm steady for 2 weeks or 3 months sourcing and negotiating leads, how do I expect the results I want?

I have a 2-acre lot on a busy freeway (85k cpd) with convenient on/off access.

Like any service based business, you're going to encounter a variety of types of fee structures.

My experience has been that summers are very busy but it falls off the rest of the year.

The LLCs need to be substantially run as their own business.

It's in a location with million dollar homes and small businesses, so the price they're asking isn't cheap.

We have experience with this exact type of rental: we know the business, we know the building, we have the contacts, we've done the management, and had other arbitrage properties pre-COVID, etc.

DSCR loans are business purpose, so you wouldn’t be able to occupy the property.