I run into this every once in a while, but the sticking point is always the mobile picture. If I want to add a todo item using my phone, what are my options?
Taskwarrior's mobile app is sufficient, though configuration for use of a taskserver is fiddly. There are cloud taskservers (I use https://inthe.am, but you can host your own, too.)
I use org-mode + orgzly on Android, synced via Dropbox.
Obviously, orgzly gives you only a small fraction of org-mode features, but you're still able to edit org files, view & search through tasks, display somewhat customized agendas and get reminders on tasks with scheduled dates and deadlines. That's enough to get me through the times when I don't have access to a computer with Emacs (or terminal & SSH, which I can use to connect to Emacs instance running on my home desktop).
I use emacs org-mode with beorg¹ on iOS. beorg has very good agenda view and allows dumping new entries. Refilling and tagging I do later with real emacs. Syncing files is done via webdav (nextcloud to be exact).
- Add a time to reminders, as they're currently only due date reminders. (Nearly useless, really, other than appearing on your calendar.)
- Merge Tasks remidners with the Google Assistant reminders, and the Google Keep reminders. Three different places to have Googley reminders, all separate.
- Re-arranging the task lists. (You can only re-arrange tasks on a list, but not the lists themselves. This is a basic feature that is surprisingly missing.)
- Add a widget for the Android home screen so you can keep a specific list in full view all the time. (This is kinda' a personal wishlist item.)
- Add Google Assistant voice command support to create tasks, review tasks, etc. (Super useful while driving and remembering something.)
- Add tagging to list items, and a search function so you can have "meta-lists" like "All Items Due Today" without having to maintain items across lists. (This would allow you implement the GTD methodology.)
- Add minimal per-list collaboration of some sort, similar to Keep. (I guess it's already available for Keep, so maybe not super relevant.)