You are sharing new information in this last post that was not in the original post.
We can agree to disagree on HDD reliability. I've got 20-year-old HDDs I recently dug out of retirement to test some hardware and they still work. And again, the bulk of "the cloud" is HDDs. Some will go bad... but that's true with SSD, optical and chiseled (on rock) storage too.
If you want to do serious video editing in a consumer rig, that's SSD and, even better, SSDs configured as RAID-0 for scratch, if not enormous banks of RAM. So with this desire in play, you should have BOTH (slow, big, long-term storage and fast storage for editing). However, I edit intense video on some big RAID-0 HDDs and as a RAID, it is plenty fast enough for my video editing. Of course, I favor SSD for even faster READ-WRITEs, but video files can get BIG and HDDs can deliver much more capacity than SSDs for big video edit files. Even with many little edits and renders of short-length video, one can fill up 4TB SSDs and need more space to finish a project. A big 16-20TB HDD leaves lots of room and a RAID of 2 or 4 of those is enough for motion picture length editing while also delivering fast READ/WRITE (faster than most editors can edit).
Unless there's been some major change in very recent years, optical is SLOW and not suitable for intense interactivity for video editing. Optical is another good, long-term storage medium. I'm not aware of fast Read-Write optical for something like video editing. Based on some perhaps dated knowledge of the 3, consumer-level video editing will most benefit from gigantic RAM, then RAID SSD, then SSD, then RAID HDD, then HDD and then Optical.
I'm quite HB knowledgable. As others have offered, there is no setting for targeting "filling a BD." HB and tools like it are generally revolving around leaving the resulting file size variable while preserving the quality as close as possible to the often gigantic MKV (or PRO-RES) original. So some files will end up big and some much smaller, even if runtime length of the originals are exactly the same. There's all kinds of reasons for this but generally, the goal is to preserve the playback quality while reducing the file size. That file size will be variable because that playback quality target is fluid. Complex video demands more storage. Simple video requires less.
You could play with settings to perhaps find a way to make a given movie compress down to just under a BD capacity. But then apply the same settings to the next movie and the resulting file size will be different. There won't be any settings that will always- say- target 94%+ of BD capacity. The only way to approximate that would be to tweak settings for every movie with best guesses, render and then adjust settings and re-render & tweak settings over and over until you find a settings combination for that ONE movie that is about 94%-99% of BD capacity. Then repeat for every other movie.
If you insist on going the Optical storage way regardless of this post, the HB strategy for that would be to find settings that render playback that meets your own judgement of "good quality" and then "bunch up" the rendered files to fit on a BD disc. A typical 2 hour movie might render well into as little as 1GB to as much as 20GB or so. If you were using BD Double Layer capacity at 50GB, a disc might be able to archive upwards of 49-50 1GB movies or 2 movies at 20GB with maybe a few small ones as "filler."
You could put optimal (for your eyes) HB renders into a folder until it is near 50GB and then burn all of them to an optical disc. Do it again for another batch. And again for another batch. Eventually, all of your movies might be on 10 optical discs or 20 or 50 depending on how many you have.
Will the Panasonic play H265 or H264 movies (rendered by HB)? Will it present you with some kind of on-screen menu when there is more than one such movie burned on a single disc? If you absolutely want playback on the Panasonic BD player, you'll need to test that to see what it will and will not support.
Or again, dump all of your HB renders into some form of big storage, hook an AppleTV to your TV and all of your movies will be available on demand without having to fetch any disc and insert it. A big HDD will hold hundreds of HB-rendered movies. One media backup HDD will avoid your "bad luck with HDD" scenario taking out your collection.
OR again, one HDD with AppleTV for easy access to all of it and all of the same renders also bunched up on Optical discs as a non-HDD backup in case bad luck with the HDD strikes again.
I do understand what you are trying to do here. But I'm pretty confident you are not going to find a time-efficient way to accomplish the objective. Nobody I'm aware of is trying to create such software to target maxing out BD capacity as you seek because- I would presume- not much of a market for that would exist. In short, you are fighting great currents here. You might be best served to alter the target and "go with the flow."