I’m no expert here, but I’m not sure Deltachat uses RSA.
“The default OpenPGP symmetric-key algorithm in Delta Chat is AES-128.”
-https://eprint.iacr.org/2024/918.pdf
“The good news is that symmetric ciphers, such as AES-GCM, are already post-quantum secure.”
-The state of the post-quantum Internet
(AES-128 is AES-GCM with a 128-bit secret key)