Yeah, i would say a moisture meter is CRITICAL when drying your own wood. Its kind of critical for anything other than kiln dried material from a reputable supplier. I have no idea how you would tell the difference between 15-18% and 10%. Certainly you can feel the difference between 25-30% and 10%, simply by the weight of the board. I have a few hundred dollar pinless meter, and i like how quickly it can check 3/4" depth on boards without leaving marks. I use it on everything now, and its a good business practice. Who is to say the kiln dried sapele didnt sit outside in an open air pole barn for months in the humid summer?
Good luck with the process. I had fun drying walnut and found it to be pretty rewarding. It would be significantly better/easier with some type of material handler, like a skid steer with forks, and a level concrete pad with a roof over it. As Jim can attest to, manually stickering 500+ bdft in one go is awful work. Especially wet 8/4+. I would do it again. Not for another couple of years, but its worth the effort.