I'm trying to get TTS working on OSM, but I read that you have to have a TTS engine outside of the app. Any recommendations for an FOSS (and private) TTS engine from f-droid?
Not from f-droid, but Piper TTS models are great and performant. You can install the apk and the app requires no permissions. They also have other models other than Piper (eg Coqui). For English, I recommend recommend vits-piper-en_US-lessac-medium for the model.
The best TTS engine I found is from the Kaldi project. They have a bunch of different pre built apks with different voices based on sherpa-onnx or piper models.