[lightsout]

What would your advice be?

SPDIF needed then get the (S905X3) Ugoos X3 Cube or an (S922) Ugoos AM6.
Quality hardware properly designed with correctly engineered passive heatsinkS.

Add a Wireless Plug n Play WeChip G20S or G30S remote to control both of them. Such packages are found on AliExpress.

The "S" version of these remotes have Infra Red Learning capabilities.

Use the existing FireTV Stick 4K for paid, copy protected video streaming Apps.

Thank you and also @FXB78!

As far as I see, the only difference between Ugoos X3 Cube and Ugoos X3 Pro is 2GB+16GB vs. 4GB+32GB.

Cube would be enough for CoreELEC from what I've read, although this would be the first proper Kodi device for me, where I would use the librabry etc., so I'm not sure what "enough" actually means here. How big can a library become? Not that those 16GB are not enough in the end. I think I have something around 500 movies and maybe 30 tv shows.

For Android 3-4GB RAM, so the Pro model, is "required" for smooth usage. Not sure if I will need Android, because I've the FireTV Stick 4k...

Is it possible (adviceable) to install CoreELEC on the internal eMMC flash or would I need an SDCard to run CE from?

Also is the original remore control that bad, so that an alternative is needed?
What are those Infra Red Learning capabilities of the "S" models of the WeChip G20/G30 remote controls? Turn the device on / off, or more?
Can't the remote control of the FireTV 4k be used for the Ugoos X3?

Never bought anything from AliExpress so far. Mixed reports about it's reputation apparently...

As far as installing coreelec you can install it to the emmc, I recommend this rom
https://forum.freaktab.com/forum/tv-play...roidtv-9-0

I have the X2 with the same rom from the same dev. It puts both Android and Coreelec on the emmc. You can switch between the two, to my surprise once you 
boot into one of the OS's you stay there even through reboots unless you give the command to boot to alternate OS.

So for me I can't even tell Android is there, its like a pure CE device, which is what I wanted.

I have also read many times on the Coreelec forum that 2gb of ram is plenty. I have also the 2gb 16gb version (but the older X2 model) and so far it is working great. 
I have maybe 800 movies. I haven't done tons of viewing with it just set it up and played some 4k HEVC stuff. It skips around nice and smooth through a 60gb mkv.