I highly recommend this passive undersaddle pickup.
https://opentosourcesensors.com/prod...saddle-pickups
The caveat is that your Tonewood amp would probably need an input impedance of at least 2 or 3Mohms for best results. Some of the passive undersaddle pickups out there require a 10 Mohm input impedance for best results.
I took a quick look at the Tonewood Amp manual but didn't see a reference to input impedance. Its probably in the neighborhood of 1Mohm if its designed to be used with mag pickups. The OTSS pickup will work with that, but it might sound a bit trebly. (You could inquire about that to OTSS's pickup designer, David Enke.)
What's puzzling me is why you believe a passive undersaddle pickup would be less susceptable to feedback than an active system. Undersaddle pickups in general are inherently a bit more feedback prone than magnetic pickups. If feedback is an issue with the Tonewood amp, that may be why its bundled with a magnetic pickup.
An advantage of the Pure Mini would be that it works well with an input impedance of 1Mohm or as little as half that in some cases. With respect to feedback issues, however, it would be more troublesome than a magnetic pickup or an undersaddle pickup.
By the way, the issue with the Baggs M1 or Baggs M80 is not that they are active pickups (in some cases). They are a little more feedback prone than a pure mag pickup because they are hybrid pickups which respond to soundboard vibrations as well as string vibrations.