As an update, I have moved beyond LiquitText for one big reason and one small reason.
Big: As mentioned by @korm the LT workspace to the side of your PDF document is proprietary and not transferable (and is a part of premium purchase). It is REALLY cool to be able to draw connections and have a free form workspace.
Small: The UI/UX, outside of the pinch pages, is not all that pleasing. I am a sucker for aesthetics, and LT does not quite cut it. They have been spending a lot of time moving cross-platform as I understand it, and therefore I felt as though my use-case was not their priority (which is fine).
Now, when I am doing a deep dive into research, I instead use MarginNote3. The biggest reason why? — I can export the mind map/outline in OPML format and use in other applications, including DEVONthink on the Mac (though not in DTTG). So, in essence, LiquidText’s only saving grace (for my use-case) is the pinch feature — which admittedly is unique and powerful, but it comes bottled with features I don’t like. My use-case demands an extreme amount of cross-app, cross-doc connections. In essence I need my PDFs, annotations, notes, etc to be my “second brain” as opposed to using this software for discrete projects.
For @AndrewM, I would just use a standard PDF editor (like MarginNote3, PDFpen, PDF Expert, PDF Viewer, or LT without the LT workspace, ), open the DT database in the Files App and make your annotations there, and they will be saved back to the DT file (you can’t start in DTTG, you MUST always start in the receiving application/editor when using iOS/iPadOS — at least for a while).