The point ot mnemonic keys is to be able to write them down easily on a piece of paper and recover them if needed, both by yourself and your family if anything happens to you. You really expect anyone to write down 100-300 random characters
There were no mnemonic at first, there were BIP32 which needed an octet string and could only produce a Base58 string that had 111 characters and was hard to write down. Then someone came up with the idea to encode that octet string as a set of words.
If you think writing down the encrypted result as Base64 (or different encodings like Base16, Base58, etc) is hard then you should focus on changing the encoding to something easier to write down instead of changing the encryption!
For example the Base64 you posted above is 80 bytes, encoding it as mnemonic is trivial, you just select a word list such as the 2048 words used by BIP39 then split the bits to small chunks that corresponds to the word list word count (11 bits) then print the corresponding words
71TjQQYPkadCq8qUA6Lqt7FhUBEjPSzgDSbBA6spbtD/j8v3JXp9Vpco0H8rS/TK2/IOMS0aHF5QIyLihGuP2dSgdoKdyDrb82O72tNPdT4=
ef54e341060f91a742abca9403a2eab7b1615011233d2ce00d26c103ab296ed0ff8fcbf7257a7d569728d07f2b4bf4cadbf20e312d1a1c5e502322e2846b8fd9d4a076829dc83adbf363bbdad34f753e
first 2 words
11101111010 10100111000
1914 1336
urban poem