The Polish sample is really good. Sounds like an audiobook recording.
By using the Whisper-derived phonetic representation (so called semantic tokens) we successfully trained a model with just a high-quality speech dataset of one language and the voice quality transferred to English.
I think you would get good results with fine-tuning but unfortunately we don't have a user-friendly notebook or script to do that right now. The biggest model is 800MB (FP32) so you won't even need a very big GPU to be able to fine-tune.
The English audiobooks are public domain recordings from LibriVox (via the LibriLight dataset).
Link to librivox for others: https://librivox.org/