[EE84M3i]

Arguably the entire reason that Rijndael won the AES competition is because it isn't "randomly mashed up". Each of the individual steps in AES is (relatively) simple and has an explainable design motivation.

Unfortunately I don't have any good advice on where to read up on designing ciphers, as I learned this in a uni course on symmetric cryptography.

[vicpara]

It is also very educative to read the AES selection methodology for the best symmetric encryption algorithm. Multiple criteria were used to find a good compromise between security margin and implementation convenience. What is also a curious thing is that Rijndael was not the algorithm that provided the largest security margin, Serpent was yet it didn't get selected.