Was really thrown by all the references to "GTD". It seems like a heavily commercialized productivity workflow?
My entire productivity system revolves around Vikunja[1]. It's essentially just a todo list application, but it's super flexible (e.g. view as list, kanban, gantt, etc), supports having many lists (I keep one per different idea/topic), and so many other features. You can also self-host it for free, which has been a completely painless process for me personally, and gives you the standard peace of mind from being in control of your personal data.
Highly suggest checking it out!