Amlogic are actively upstreaming support for their "newer" chips (since SM1) but their developers are inexperienced at upstreaming to the mainline kernel and indoctrinated on their BSP codebase which "works, but don't ask how" so the initial state of all patches is poor and it takes many slow iterations to raise them to a mergeable standard. In the last week they started to upstream hardware decode support (to supersede the long-stalled upstream 'staging' driver effort) but this is going to be a very long slog.
So I'm confident S905X5M will be supported, but don't ask me which year this will happen in, and please least limit yourself to asking for an update only every six months. And don't be offended when the answer is copy/pasted from the previous question 
RPi5 (my family daily driver) is a considerable performance bump over the RPi4. Rockchip RK3588 does not support VP9 hardware decode yet but like RPi5 has a ton of CPU. I would expect it and RK3576 to be nice boards in the mid-term future (long before the newer Amlogic stuff matures). There are lots of inexpensive Intel x86_64 devices these days too.