I’ve been using Things 3 for about a year and I’ve noticed a particular pain point. It’s annoying to replace tags.
A lot of my tags are to do with a task’s progression.
For example, some tasks just need doing. Other tasks are captured as a kernel of an idea and need planning before they are done. So a task might start tagged as Plan, and progress into Do. (The practical application of this, is that it allows me to quickly find tasks when I feel like “hey, I’m mentally drained right now, but still have physical energy sufficient to catch up on some mindless errands”. Or, conversely, “I have no physical energy right now, but plenty of mental energy to plan or research something”.)
Or, I might tag a task as <1h in estimated duration. Then, on review, I might realize it’s a smaller task than I thought, and change it to <30m.
Rather than have to remove the old tags, I’d love to just change a field value, like when I reschedule a task.
I am not sure, if I understand you right, and I have no deeper experience with Things3, so I do not know how large the differences are, but I use Omnifocus as my TaskManager for a longer period of time now, and you could change the Tags there very easy by just deleting the old one, and setting the new. There is also a regular field for setting an estimated duration for a task, that could also be easily altered, if needed.
So maybe it is worth for you to have a look onto Omnifocus, who are offering also a free trial period.
Also, in OmniFocus you can configure perspectives (think "saved searches) that look tasks with durations less than <x>. Here are some of the parameters that perspectives can search on. Searches can combine multiple parameters.
For true custom fields, my mind goes to the more freeform kanban/table-style apps like Trello, Notion, Asana and ClickUp. A more DIY approach like Retool or TapForms would also provide that. I’m sure I’m forgetting something that looks more like a traditional task app and/or traditional Mac app, though.
One way to speed tag assignment and removal in Things is to use built-in keyboard shortcuts. In the Tags window, you can assign a tag to a letter or number. I frequently use a half dozen or so tags that have a keyboard shortcut. Example: home=h, work=w. Typing control-h assigns the “home” tag. Typing it again removes it. Typing option-control-h sets the view to items tagged “home”.