long story short when you use the minimum number of seed words (12 words) you are generating a master "key" which has the same strength as when you generate 1 private "key" and that strength is 128. so using more words and gaining more strength is not really making a difference if you consider 12 words weak then you are considering bitcoin private keys weak.
(secp256k1 curve is a 256 bit curve with 128 bit strength. read SEC 2: Recommended Elliptic Curve Domain Parameters; Table 1)
What about key derivation. What are the chances that one could derive a seed by taking control of a public master key and something else?
as far as i know if you are not using hardened key (which most wallets are using them) it is possible to find the master private key by having master public key and 1 private key.