Is the Notes app to simple as a substitute for DevonThink? I’m also hesitating on upgrading to the latest version.
For Writing, I particularly don’t like the idea of moving to a paid-subscription format like Ulysses or Bear, but those two seem the most popular options that have compatibility with Markdown. I double on the suggestion of Manuscrits (which is now open-source), or AI writer.
This blog might also help (?) https://appademic.tech/