What is your network infrastructure that is giving you those poor performance numbers?
Most consumer all in one routers are crap but not that bad. the file server should always connect to the main hub of the network with Ethernet (whether that be the router, a switch or an all-in-one crap box), these days pretty much everything should be at least 1gigabit.
Are you trying to use wifi for everything? that's a recipe for disaster unless you really know what you are doing and have multiple APs and careful signal strength and channel management
The shield pro 2019 is probably still the best overall, it's not perfect as there are some weaknesses due to the age of its chipset, but for all the common formats used in Movies and TV it works perfectly, especially if you are playing full remux files, not re-encoded compressed video. Kodi runs very well, Plex runs very well, Jellyfin is mostly perfect too, but has some limitations in the current version.
Yes it supports HDR10 (not10+) and Dolby Vision, which covers 98% of all 4K blurays and TV shows, anything HDR10+ just gets played in HDR10 compatibility mode, if you TV doesn't do DV it plays the HDR10 layer on 99% of files. There are some issues with HLG as it isnt properly supported but you don't come across that format all that often and there is usually an SDR or regular HDR version available, if your TV supports manually activating HLG then it works fine.
Yes there is a minor colour bug in some DV content, no it isn't the end of the world as some people make it out to be.
It is one of the only players that will give you full DTS:X and Dolby Atmos support, it has a very nice configurable upscaler for lower res content (AI upscale on low works excellently with minimal artifacts), it still has a lot going for it despite its age.
Also its easy to decrapify with ADB, you can easily configure third party launchers and other fun stuff.