I just came by Obsidian app which is a note taking application available on all platforms. I have been using DEVONthink Pro since 2018. I bought the mac and iOS apps mainly because it offers seamless offline syncing between these two apps. I can write my journal on iPhone and sync it to mac all offline. It's a bit involved but not that complicated to setup offline sync. The main issue that I face now with DEVONthink is that mine is version 2 and now the latest supported version is 3. Version 2 is no longer supported and the app crashes on iPhone when searching for anything.
I found that Obsidian is really good for writing notes. Notes are saved as markdown (.md) files in the local file system. These appear when we connect iPhone to the mac and can be easily backed up by dragging and dropping the folder. I read that there are plugins for syncing but a simple copy works for me. Also, the Obsidian app is free for personal use.
Update on 15 June 2023
I have migrated all my notes from DEVONthink to Obsidian. Now the app is taking more than 20 seconds to open an offline vault containing 283 plain markdown files. This is unusable for me. So I moved away from Obsidian to Working Copy.