You are all funny guys here....

FCT or FCL for more than 90 minutes seems to be very boring for me.
No idea if you would get a day room in the FCL or FCT, but even if so, this is not great for other travellers if you occupy one of the rooms for the full day... And furthermore there is no window in it, so why would you do that... You dont want to be in a prison when you are travelling F?

Sometimes I think O class travellers have strange ideas about maximising.....

I would do it very differently:
Make a reservation for the Kempinski, it will be free of charge.
Once you have deboarded, go to the place where the shuttle leaves and call the hotel. They will arrange a pickup for you, also free of charge.
Spend the day in the Kempinski. Have a nice meal there (it is a bit costly, but really great quality in "Forsthaus" or eat something in the "Wintergarten").
Go to the pool area, they have an outdoor pool and an indoor pool.
The outdoor pool is great, even if it is snowing outside, because it is heated.

It is one of the greatest places around, and you will enjoy it free if charge.

You can also go for a walk in the forest, and the rooms are very nice.

I dont like the staff in the hotel, so I would not go there if it is not free of charge. But in this case - it is just great!!

You will be able to stay at least 8 hours in the hotel, maybe even 10. So you can swim, make some sports if you want, watch a nice movie, sleep a bit and have fresh air.
This is really maximising, because it maximises your quality of life. Lounge maximising is bad for everyone, even for you. Kempinski is a great thing on the other hand. I would go there even if I only had 5 or 6 hours of layover. Just because it is within the forest, fresh air and great surroundings.

Dont forget to make a reservation for the shuttle back to the airport asap when you arrive, because they are fully booked sometimes.

Enjoy!