Just stay within the valid H.265 profiles and level, which hopefully x265 will not break by default. Most newer GPUs can even decode 8K content, which means support for up to level 6.2, older ones may be limited to 4K, which would be level 5.2, but x265 should set the level value on its own, instead of requiring you to override it.
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LAV Filters - open source ffmpeg based media splitter and decoders
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