saying you don't understand how something that is created by experts and reviewed by other experts and then not using it and instead using another thing you yourself create is just crazy. it is like saying i don't understand how airplanes work so i never get on one and build my own plane and use that while not having a clue what i am building.

worst thing you can do is also advise others to do the same!
most of the times when people think they've come up with their own "encryption" system they are either not adding any kind of meaningful security while doing the so called "encryption" or they have created something irreversible. i have seen so many users who did crazy things to their mnemonics and now after a couple of years they can't remember it and all their bitcoins are lost for good.

even if you are paranoid there are still better solutions. for a mnemonic (which is quite simply human readable encoding of an entropy) you can use a coin to flop or a 16 sided die to roll and create that entropy.
for storage and encryption there are secure encryption methods such as AES-256 that should be used.
anything else is most probably going to either be weak or could even be irreversible.