Wow, well a lot of questions...Can i ask what format are you recording your videos at, and what is your current editing software, as i am just curious why you don't want to use the new HEVC recording mode.
Also, why do you need to install codecs to play HEVC files, every TV now will play HEVC files, the same as every laptop or computer using VLC or other media players will also play HEVC without the need to install codecs, actually nobody has had to install any sort of codecs on a computer or laptop for years now, so maybe you are still using an old computer.
I love HEVC (h265) and i won't ever go back to using AVC, it records the same quality video at 35% less file size than the old AVC (h264) format, and for me that is a game changer right there.
Also, you do not have to go out and buy expensive software to edit HEVC, most cheap editing tools will do it now days, so obviously you have not looked very hard, and if your editing software cannot open or edit HEVC files then it must be one of those cheapies, or some freeware version, and possibly old software as well, so maybe it is time to upgrade that as well.
I currently record in 2.7k/60p on my Hero 7 because it has Hypersmooth, where 4k/60p does not, the Hero 9 will do 4k/60p with Hypersmooth and that is also the game breaker for me as well.
I agree that 5k is pointless on such a camera, but you also need to understand more about video resolutions and why many people want to record at the highest possible resolution, as it allows them to shoot at a high resolution, then they will export at a lower resolution, but the high resolution source file allows them to crop all of, or parts of the source file before exporting.
I would highly recommend that you go buy a license for a software called Video ReDo you can get the TV Suite version 6, or the Professional version 6 that i use.
These will import HEVC and AVC files from a GoPro, you can cut bits out, add titles etc, and join the file back together without the need to re-encode the video file.
I suggest that you download the trial version of TV Suite first and try it, there may be a limitation on the file duration for the trial version, but it should give you an idea of what it is like, you can buy it for us$95 after that, but in my opinion it is the best non professional editing software on the market, they even have a forum, and the license is for a lifetime.
Also, did you know that if you record in HEVC you can use a free open source encoder called Handbrake to transcode the HEVC files to your preferred AVC format so you can then edit them using your current editing software.
Sometimes in life you just need to move with the times, for most people HEVC is the new game changer in video, and maybe it is time that you moved on as well.