Like others have said - it depends on the source media. In general, grainy sources require more bitrate to achieve a given quality as opposed to a clean, digitally shot source. You can choose a random bitrate and encode all your sources with it but you might not like the results or your encodes will be bloated for no reason.

Personally, having used both x264 and x265, I would stick with x264 for 1080p content. Yes there are some space saving advantages to using x265 but the time it takes really just isn't worth it - in my opinion. This is assuming you're using software encoding and not NVENC or QuickSync. Hardware encoding is much faster but yield larger file sizes and lower quality when compared to software encoding - again, not really worth it in my opinion.

Just curious as to why you're looking to store that much data on a high capacity Blu-ray disc (BDXL) as opposed to flash storage, SSD or a hard disk. The medium, as you've come to find out, is expensive and from what I understand you need a specific optical drive to read BDXL media. I know these discs use a non-organic material but I question how resistant it really is to the elements. Disc rot, though not common by any means but much more common than bitrot I feel, is something I'd try to avoid.