First, I'll grab an explanation between X.265 and H.265 from the web:
H.265 is the video compression standard, while x265 is the encoder that implements this standard. In simpler terms, H.265 is the "what" (the standard), and x265 is the "how" (the tool used to encode video according to that standard)2. H.265 offers better compression rates compared to H.264, making it more efficient for video streaming and storage
The same applies to H.264.

Now, your trick play with your TV playback. I tested my LG Nano85, and had no issue FF with a 4K/H.265 video. But when I tried the same thing on a 1080/H.264 file, it just sat and spun and never resumed playback. In either case (your problem and mine), it's the fault of the TV, not Serviio. TVs are terrible DLNA players unfortunately, and I quit trying to use the built-in TV player many years ago. I instead use either Roku players, the Nvidia Shield (my main player since it can play almost anything without transcoding) or the latest version of the Fire TV Cube (which can also play almost anything).
If I had to suggest a separate player, I recommend either the Fire TV Cube or the Nvidia Shield. In both cases install Kodi as the player to use, and you have the ability to send the audio from your media directly to an AVR (AV Receiver) so you can get the lossless audio that most 1080 and higher discs contain. The Roku will not passthrough the lossless audio, so it needs to be transcoded to basic Dolby Digital.
One caution about the Shield. The most current version was released 5 years ago, so it's lacking a couple of things that might be important to some. It doesn't support HDR on YouTube, nor does it support the AC-4 audio codec that is getting close to becoming available on some streaming sites. Roku does have a player (the Ultra) that supports both. I can't say if the Cube does.
Dan
LG NANO85 4K TV, Samsung JU7100 4K TV, Sony BDP-S3500, Sharp 4K Roku TV, Insignia Roku TV, Roku Ultra, Premiere and Stick, Nvidia Shield, Yamaha RX-V583 AVR.
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