Tvheadend

Data will flow:

Recording - Tuner -> TVH -> NAS
Streaming - Tuner -> TVH > Kodi
Playing recording - NAS -> TVH -> Kodi

TVH can't tell the difference between local HDD and network shares, so there is nothing stored on local HDD before being sent to NAS.
With gigabit ethernet your proposed scenario will be fine.
While theoretically setting up TVH so that it can record to local HDD would improve efficiency [one less trip over network], you will be far, far short of saturating your network and in practice this unlikely to make any difference.

Thanks for the answer.

I guess I can first test by having recording share on the NAS and run Tvheadend on NUC. I do not see a lot of negatives except one. If I do it like this, I need NUC to be on all the time. I cannot power down NUC. NAS will be on all the time anyway so running Tvheadend on it would not make a difference in terms of power consumption while it would let me shutdown NUC whenever I don't use it.

However having Tvheadend on NUC allows me to run timeshift directly on NUC.

I will have to test both and see what is better.

Put solar panel on roof do what you want with your own energy

There is always a positives and negatives, there is no universal solutions'.
NUC idle is less than 10W (3.6-11W) claiming Intel
On other side this is equal to 3.5" HDD consumption.