N26 is PSD2 compliant [1] and they seem to have a sandbox available [2]. I can't speak to the dev experience but I like them as a customer.
[1]: https://support.n26.com/en-eu/security/open-banking-psd2/psd...
[2]: https://github.com/n26/psd2-tpp-docs/blob/main/doc/sandbox.m...