I'm advising a back disc brake too. There's quite a lot of stopping power in a front disc only, be prepared to endo.
Adding a back brake won't (indeed, cannot) add any braking power to something which can already endo.
If i did get OTP 29er wheels, how easy is it to swap the QR axle for a bolt on?
Shimano rear axles on their lower end rears are 10×1 threaded, so you can easily swap to a generic solid axle and track nuts. Fronts are 9×1 for conventional dropouts, not such an easy OTP solution but that FireEye axle could have the threads extended to take the cones and locknuts. Making bolt-on axles for QR15 front hubs is pretty simple too, wouldn't even need the spacers I used on my tandem.
Adding a back brake won't (indeed, cannot) add any braking power to something which can already endo.
Shimano rear axles on their lower end rears are 10×1 threaded, so you can easily swap to a generic solid axle and track nuts. Fronts are 9×1 for conventional dropouts, not such an easy OTP solution but that FireEye axle could have the threads extended to take the cones and locknuts. Making bolt-on axles for QR15 front hubs is pretty simple too, wouldn't even need the spacers I used on my tandem.