It sounds reasonable to me.

Since you're writing/reading all of the subcolumns at the same time, I would opt for a standard column with the tags serialized into a column value.

I don't think you need to worry about row lengths here.

Depending on the reminder size and how many times it's likely to be repeated in the timeline, you could explore denormalizing a bit more by storing the reminders in the timelines themselves, perhaps with a separate row per (user, tag) combination.  This would cut down on your seeks quite a bit, but it may not be necessary at this point (or at all).