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I use https://steemthat.com/steemthat-direct-message-on-steemit/

I can't guarantee its safety with your active key but it seems legit and you need zero coding knowledge. Its only suitable for 10 accounts at a time and way more manual than code but I spent 4 hours on linux trying to do that and got close but in the end I decided it wasn't worth my time

Mass memos? You can do that with:

https://github.com/AdamCox9/Steemit-Follow-Bot/blob/master/send_memo.js

Add an array of user ids and the message in the config.js file.