Edit: Oh also want to mention that I generally prefer privacy oriented apps - so if there is something, even paid, that will keep my notes on device without sharing them with a server I'd love to hear about it.
I'm on an iPhone and I use Streaks https://apps.apple.com/us/app/streaks/id963034692 . I really dig it. It's easy to set up, easy to add or pause tasks, and easy to correct past mistakes (I often forget to record I took my meds even though I can see I remembered by looking at my pill organizer, fixing this takes a couple taps).
You can configure it to remind you to do a task N days a week. I use this feature to track checking the mail. You can also make a daily task and configure it with a "2 day rule" that gives you a little wiggle room. I want to read a book and practice chess every day but sometimes I miss a day -- this rule lets me miss one day but not two days in a row.
It's well designed with lots of color choices and icons for each task, plus it has Apple watch support. I'm pretty sure it's all entirely offline other than backups to iCloud, which can be disabled.
Finally, it's a one time purchase. If it wasn't I wouldn't have given it a try.
And for someone who likes to "work on their productivity", it's a never ending swamp of plugins and optimising data views and reports.
I just switched to Things3 and couldn't be happier. It has very little customisation and options. It just lets me write down my TODO-lists.