Aside from the airline mileage programs, there are 3 major rewards programs that have transferable points: Amex Membership Rewards, Chase Ultimate Rewards, and SPG. Amex and Chase transfer 1:1 into many partner airline/hotel programs, and SPG transfers 20K:25K into many airline programs. After you transfer points from these programs to airline/hotel, they are essentially stuck there (without costly transfer prices to other programs).

You can get multiple sign up bonuses within each of these programs to amass large number of points. Also, you can combine that with sign up bonuses on specific airline cards to get the miles you need. For example, Chase United 55K sign up offer + Chase Sapphire Preferred 45K sign up offer. Chase points transfer to United, so you have 100K from those 2 sign ups.

The other thing you need to consider is the cost of your flights. For BOS/JFK-IST, United business will cost you 115K miles per person if you can find availability on United or 140K miles per person on their partners. A cheaper option would be Aeroplan (Air Canada), which is a transfer partner of Amex MR and SPG. Aeroplan charges 105K for business round trip BOS/JFK-IST. You do have to be careful about which airline you're flying on as some will (Lufthansa and Air Canada, for example) will result in large fuel surcharges associated with your award ticket. However, you could use the Aeroplan miles to fly your route on United, Swiss, Brussels, and most importantly, Turkish, for the cost of 105K miles + ~$50 taxes/fees per person.

For regular spend, consider that at worst you can earn 100K Chase/Amex points by using one of their cards. The Chase Sapphire Preferred earns 2 points per dollar on restaurants and travel, while the Amex Premier Rewards Gold earns 3 points per dollar on airfare and 2 points per dollar on grocery. The SPG card effectively earns 1.25 miles per dollar on everything. So with an optimal mix you can get well over 1 mile per dollar on average.