Is it too optimistic to buy a Hobonichi planner for 2021?

Possibly. I burned my 2020 planner in Aug

Probably. I have given up on planners. Half the time I end up emailing myself to remember things so the planner collects dust.

I have an undated Penny Pages planner that I have been using

Not necessarily you could still use it to track things such as water intake and exercise. Not every day has to be an adventure to write down you can write down little things that made you happy that day or something that made you smile