The things widgets are fine. I’d be very frustrated if Things didn’t update to use new headlining Apple features given their historical trend of being a well integrated app.
However, I believe Todoist has also upgraded to Lock Screen widgets based on an article I read about early ios16 capable apps. Not sure why you’d bother switching when Todoist also has this new feature set
Is there evidence that they are working on Things 4? In Things 3, shortcuts integration has been sparse and there’s almost no filtering of items (or smart lists). Reminders is looking more enticing, but it doesn’t have the ease and simplicity of Things 3.
I love Things and need it for my complex paid work on a computer, but I’ve started to integrate Reminders for personal stuff and a job I do very part time where I have to use Windows so the speed of Reminders on my phone for simple stuff/shortcuts/better share sheet is super handy in those contexts.