IMO the main reason to go for 4K is the HDR color accuracy.
Most movies are mastered in 1080p regardless, not to mention CGI effects are still mostly rendered at 2K.
But DCI-P3? If your screen can cover the color gamut or mostly cover it (90%+) then I’d go for it.
Personally I just have Radarr setup for 4K HDR and Sonarr setup for 1080p. Jellyfin transcodes 4K to 1080p and HDR to SDR just fine for my purposes (note that I have an Intel N100 box with QSV hardware transcoding and tonemapping setup)