Hello Andy, I see what appears a malar streak, too (streaking, when I as a non native speaker, interpret this correct, consists of multiple streaks)
Please compare this Trush Nightingale with a prominent (and broad) malar streak:
https://flic.kr/p/22q59jr (11.05.2016, NE-Germany), all pictures:
https://flic.kr/s/aHsmbGhwT2 . An "ID-book"-Trush Nightingale has a narrower one, the OP bird is quite typical in this respect.
Also, the pattern of the undertail-coverts seems good for Thrush Nightingale.
I hesitate to ID this bird, as I have no experience with eastern ssp of Common Nightingale and its nearly impossible to judge pattern and colour of breast and lower throat area. Here is Common Nightingale with a darker than normal breast, but this area is uniform and unpatternd (should be streaked or at least with some cloudy smuding in Thrush Nightingale):
https://flic.kr/s/aHsmNXnRif (10.05.2020, NE-Germany). Or this bird, that is darker and colder than a normal Common Nightingale:
https://flic.kr/s/aHsmQJdA4Q (13.04.2020, SW-Germany ), with many thanks to Christian Stohl, Julia Bayer and Heino Hauf for help with identification of this bird.
Conclusion? Its one of those threads that I learn from, but I hope, my post is still an usefull contribution. And I agree with all, that picture quality might prevent a safe ID (for example, is the pattern of the undertail-coverts real or an artefact?)