Even at 5'10", I like the 650c wheel size for tri/TT bikes, track bikes, crit bikes, and climbing (road) bikes. Just solves a lot of fit issues with extreme positions, and the lower moment of inertia means they spin up faster during accelerations. I've never had problems getting rims, but as you have seen tire sizes can be limited. I've seen one option at 25mm on J&B, the rest are all 23mm. Going to a MTB 26" is an option, but as has been said, disc brakes would be needed to be able to swap between sizes if you want to go to using both in the same frame.

650bx42mm has about the same outside diameter as 700cx23mm, so you haven't really gained anything by going up in wheel size.