My experiment with using DEVONthink to store my personal journal came to an end today. I exported all my Markdown files out and back into an iCloud Documents folder, and will be using iA Writer (plus a little help from Hazel on the Mac for archiving) in future.
Unfortunately, using DEVONthink (and DEVONthink To Go on the iPad) was introducing too much friction into the journalling process. With iA Writer, I can start a new journal page on whatever device I have to hand, and continue throughout the day.
It hasn't been a totally wasted effort, however — I've learnt a lot more about how to use DEVONthink as a result of this experiment.
I may end up using DEVONthink indirectly, by having it index my journal archive, in order to see what it can tell me about my writing.