

In my opinion the ideal x265 size/speed/quality is using a tuned slow preset, perhaps with filtering if the source is grainy. A test encode or few should be done to determine an ideal CRF per source.
Since you don’t seem very familiar with x265, I would just stick with the defaults in slow preset, but consider using aq-mode 3 or 5 (only available in the patman mod). You can also adjust the aq-strength to help control the resulting size somewhat, I wouldn’t go lower than 0.5.
CRF 18 is usually where I start at, but that is focusing on quality. I’d imagine you would want a higher starting point.
If you want to find out what different options are or do, the docs are a good place to start: https://x265.readthedocs.io/en/master/cli.html