Kodi + NAS + Fire TV

Hi,
I am a kind of new to Kodi, so sorry for any nooby stuff I will write.

I used for years my WD TV live to watch movies/ sitcoms that are stored on my NAS (Synology DS 212).

For half a year now, I have the Amazon Fire TV (2nd Generation) and I use Kodi just to get access to the NAS via SMB. That works fine. I dit not cofigure anything in Kodi, so no covers or … Kodi is installed on the Fire TV, not on the NAS.
Now, I want more … Covers and …

My question: where do I install Kodi? On the NAS, on the Fire TV ? Or everywhere: NAS; Fire TV, computer, mobile phone (Android) …?

I have cat7 LAN cables in each room. No need for wireless stuff. I also watch movies on my desktop (Windows). That means I could install Kodi there first and configure everything first…

I am not sure about installting Kodi on my NAS (DS212 with WD Red); the NAS is now more than 5years old (1,3 GigHz sincle core, 256 MB Ram (no option get more RAM))…

Hopefully somebody can help me with how start my configuration…

Thank you…