* ODROID C2 and the Rockchip dev. boards.... all of them do not have SPDIF embedded into the board. The ODROID C2 is pretty damn stable these days tho.
* Minix NEO U9-H - the CoreELEC team are supporting the S912 chipset in it with their stable Linux Kodi Leia release. The packaged MINIX A2 lite mini Keyboard remote that some U9's come with is a beauty for long term Kodi usage.
* Vero 4K+ has dedicated after sales support direct from the developers of the OSMC OS. The Vero 4K guys are talking about "5 years of software updates" in their advertising material.
* Rockchip boards at the moment only really have one LibreELEC developer supporting them (@kwiboo)
Stable mainline Linux Kernel like the current (v 4.19+) RPi is running - none of them have that currently if you want long term support.
Talk to us in about 1 years time - the next upcoming Kodi v19 "M" will dictate which board works better vs the rest.
The RK3328 / RK3399 boards come the closest at the moment BUT they currently do not have HD audio passthrough support when running Kodi Leia.
What are a bunch of users doing in the interim ?
Buying something cheap like a generic, pretty snappy AMLogic S912 box. Run CoreELEc Kodi Leia on it... and then use a mini USB receiver dongle Wireless remote with it like the MINIX A2 lite. Such a remote will be highly compatible no matter what device you plug it into, in the future.
Have a read of this:
S912: Beelink or Vorke/Tanix (click)
Long term support - I suspect the Apple TV 4K will last a long time, you use something like the Infuse or MrMC Apps (see the review). Yes I know it does not even run Kodi.
ATV 4K really is quite powerful already and better future proofed vs anything else I can see. Apple tend to support their products for a relatively long time vs the competition.
5.1 / 7.1 Dolby TrueHD and 5.1 / 7.1 DTS-HD MA is losslessly decoded to 7.1 LPCM on the ATV 4K for no loss in audio quality.
Long term support by a dedicated company or bunch of developers is going to cost $$$ no matter which way you slice it.