I'd do the United Explorer before the Chase Sapphire or Ink Bold because 50,000 United bonuses don't come around often.

And you get the miles after your 1st purchase!

You may want to get the Southwest business card with the United card, because it may be easier to negotiate 1 business and 1 personal card application from Chase instead of 2 business or 2 personal applications.

I'd also consider the Citi Thank You Premier for 50K points or $665 in airfare or $500 in gift cards.

Your targeted AMEX offer is good. There was a publicly available offer last year for 1 day where you could get 75,000 points for a $10,000 minimum spend.

Good luck!