You should look into an industrial machine, like a Consew or industrial Juki. They have models with harps at least 16". You need to do some homework, however. They are often geared to heavy duty sewing like multiple layers of canvas etc. The feed dogs could be too aggressive and can tear quilting cottons. Many have built in walking feet, but may have the same issue. Also, make sure you understand what the pricing is. Many models don't have integrated motors. The motors are mounted on the underside of the table (which you will also need). So you may have 3 separate costs. They may be geared to sew very very fast. this can possibly be changed with a different drive gear or a rheostat.
I know you are not looking for long arm advice, but it does sound like you have a problem machine. Unless I change something (thread weight or batt type), I don't have to play with the tension or even monitor it as closely as you state.
Last edited by PaperPrincess; 11-30-2015 at 03:07 PM.