Notebooks
I’m not sure there is a single app that fills all those needs – especially one that does reminders and tasks. Notebooks comes to mind – it does have a basic task list.
Keep IT
This thread focused on Keep It.
Use what you’ve got
If you want to give up DEVONthink to avoid a subscription, you’ll have to pay for the other two options mentioned above. Or you could achieve all your requirements with the kit you probably already have:
iCloud for file sync across devices.
Notes and Calendar and Reminders for the notes, reminders and tasks.
Brainstorming – not sure how you’ve defined that for yourself.