I am just wondering about sturdiness and longevity?
I guess depends on the degree of usage.

It would probably not be used that much. But perhaps I will try to test it out at least and see how it works. Either way I will look for a more sturdy motor.

 

You will not find an adapter/coupling that will fill your requirements for sale, anywhere.
You must create it yourself.
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bbq spits use 8mm square shafts. Slide right into the motor. Just cut a section of the square spit shaft. Here is motor only with 8mm female (shaftless). You can also buy sets with motor and spit. I don't know if you planned bi-directional or what voltage you're after but this is AC. There is a small-ish motor inside that runs at higher RPM. Ive seen Dc motors that will likely substitute the size of the motor if you want to use this gearbox and hack your own.

https://www.thebbqstore.com.au/p/10kg-electric-240v-rotisserie-motor-suits-8mm-squa/EM-2010

 

The BBQ motors are typically shaded pole, which makes them non-reversible.