Best x265 software and time/quality settings?

[YaBoyShredderson]

Im thinking of changing my bluray encodes from x264 in handbrake to x265 to squeeze out a little more space.
In handbrake with x264 i use crf 18, very slow, film tune, high profile, level 4.1. Audio i set it to ac3 5.1 640kbs.

I was thinking of using staxrip instead as its what i use for my 4k blurays, but im still a little confused with it. Im thinking of using 10bit too, and with 2 pass abr instead of crf as with my current settings, the file sizes can vary so wildy, and some files actually get bigger. Im think of using 7mbits, which im aware is a bit high for x265, but i dont mind bloating the easilt compressed stuff a bit, at least i know the quality will be practically lossless. The harder to compress stuff, where 7mbits can be considered a "low" bitrate, will benefit from x265s efficency and wont be stupid artifacty.

Im not as familiar with x265 as i am with x264, and i have a couple of questions. In terms of preset, the general wisfom is to go as slow as one can bare, but im wondering what the difference is between slow and slower mainly, im not too concerned about the others. With x264 veryslow i can encode faster than realtime pretty much, 25-30fps give or take, and thats with limiting thread count too. Im asking if theres is maybe a 1-2% improvment in efficency, at like half the speed, and is therefore not worth it, that kind of question.

I also want to know if i should leave tune at none, as there is no film tune. For my 4k stuff, i essentially set the same settings as film tune manually, but i cant remember what it was. Also, what about thred count? I put threads=18 in the advanced options in handbrake, as i have 12c/24t 5900x, and opening 36 threads is a lot, as the quality loss begins to become noticable at that point. Im thinking, as x265 equals thread count, rather than 1.5x like x264, i should set frame threads to 12, and run 2 encodes at the same time? Or would it be worth going lower, how much difference in quality would it make, or is it just not relevant with 12 threads?

Also, what level should i use? If i leave it on automatic, i can only use main, not high for x265. For x264, level 4 was generally what i needed, it only mentioned bitrates and resolutuons n stuff, 25mbits for 1080p which was all i really need. For x265, i read that 12 mbits is the max for 4.0, which seems quite low, but that 30 was the max if i use high? Also, i noticed it was mentioning something to do with luma and stuff, which i dont understand, and i used 5.1 with my 4k stuff to avoid messing with the hdr.

Also, audio. I use ac3 640kbs for 5.1 as ac3 is a very old inneficient codec, and i was told thay anything less than max can make the difference noticable, but i may as well save space on audio too. What is the "HEVC" of audio coding? I want something that would also generally work with a soundbar or sound system, as i want to get one in the future. I was thinking aac or opus. I heard opus is very efficent. Whst bitratw should i use? 384? 512?

Thanks.