What 650B tires are you using? If I ever build up the one 650B Velocity Aerohead OC rim I have, I want to avoid mounting those tire models.

As to the original poster's question, I really think you should get those better 26" rims which are hopefully wide enough to mount some wider 559/26" tires. This size is so much more popular that you have numerous choices of wider tires (staying within the ETRTO size limits for your rim's inner width). How wide a 26" tire will fit and how large a diameter (larger tire on given rim = larger diameter wheel)? Could you go all the way up to a 2" (50 mm) Schwalbe Big Apple or perhaps something a bit smaller? Bit smaller include Schwalbe Kojaks, Continental Sport Contacts (1.3" or 1.6"), etc.

If you ride at night, both the Sport Contacts and the Big Apples are available with reflective sidewalls that make you instantly recognizable as a BICYCLE at quite some distance in a motor vehicle's headlights.

You also might check you are using reasonable tire pressure front and rear for your all-up weight and tire size. For some guidelines on tire inflation pressure for given loads on tires, see HERE, HERE and HERE.