[daydr3amer]

You can add an external usb stick to gain extra storage for the fire stick. Troypoint is a thing to google, but it comes with a lot of other crap trying to get you to buy a vpn. You need an OTG cable & usb stick.
I don't know if the Odroid would be powerful enough, you could ask at the CE forum. although it is the most powerful amlogic device at this time. 

Android TV device dual booting with CE-Kodi and Android TV-Netflix,  these android devices do not include single board computers like the Odroid or N2 they don't have android. EG X96 Max + the android from these devices won't run 4K netflix as they are not certified.

Hmm, and the affordable NASes come with 1GB RAM only. Additionally the VM would need access to the USB port for the tvheadend tv card, and I don't know if thats even possible.. Seems like custom is still the way to go

a nas with 1GB ram is at the bottom end of the market, VM's are only on the higher end. also, to run a tvh server from a nas, you need a compatible usb tuner, there are not many. HD homerun would probably be ok. BUT you need to check that there is a TVH server package for the particular nas. 

I am not a fan of Single board computers, by the time you have added remote, or dongle, case psu etc. 

I am running a X96 Max +. I have installed CE to sd card. It uses a S905X3 this does have temp issues, although I don't. 

With CE you can run a TVH server & client, using a usb tv tuner, or HD homerun (not cheap)

The X96 max is relatively cheap, I am not recommending it. 

A box with S922X is better but more expensive.

Since a NAS with VM support is way too overpriced for my purpose (starting from 600-800€) what about the idea running something like FreeNAS alongside TVHeaded, Sonarr, Radarr, SABnzbd on a RPi or ORDROID and Kodi on the FireTV? I think that could be the best solution to running everything on one device