[TimeZone]

is this skin suitable for FireStick4k or considered too 'heavy'? is anyone running on Fire devices, or Odroid N2 which I'm also considering

Yes, the skin is suitable for FireStick4K. It has simple butttons to turn off "heavy" features. Infact, they are off by default. The features are categorized as "All devices", "Powerful devices" and "Most Powerful Devices". So you can choose what features to turn on and off depending on your device capabilities.

I am running the skin on TCL AndroidTV (the actual TV not an android box). It is less powerful than Odroid N2 and has only 2GB of RAM and 32bit processor. I haven't tried it on FireStick yet but I believe it will do well.
 

based on what I read it works fully in Nexus v20?

Yes, it has fully been updated for Kodi Nexus and takes advantage of Nexus new features like showing HDR type (HDR10, HLG, DV, HDR10+) with their correct logo. Ensure you have the latest skin version 6.0.0 or newer installed.
 

are there mods etc to consider?

There are no mods that I know of yet but the skinner has extra features if you become a sponsor on Patreon. He adds new features frequently for sponsors and some of them eventually end up in the regular skin. Some of the added features includes support for Movie extras (gag reels, behind the scenes etc), Album extras (artist interviews, music videos etc), use a vertical menu among others. I have personally found the Movie and Album extras to be very useful, it is far much better than using the Extras addon.