For Android a hidden gem in terms of tts engines i have found is Sherpa-onnx. They have a page with pre-built apks here, if you can stomach finding your way in a huge list and looking for the specific architecture of your phone cpu. The voices that they offer are the best i have heard for an open source project. Still the are downsides. It is heavy on the hardware and will barely run on low end phones with long pauses between the phrases. Also you can only have installed one voice at a time. There is a huggingface space where you can try different voices in browser without installing anything here.