Do you want only 2 devices of the 10 your family possibly owns to work?
Do you want your family to complain that jellyfin “isn’t as good as Netflix/Disney+/etc…” Because it constantly stops to buffer and a can’t keep up the framerate?
It is completely fine if you are single and have 1-2 devices that work with AV1 and h.265 client side and that is all you need, then you don’t have to bother with transcoding at all. When you start letting other people into it, compatibility becomes an issue.
As for storing it beforehand, the entire point of AV1 and HEVC is to significantly reduce the size on disk. If you have to store 10 versions or each file, 5 resolutions each, half h.264, then you are taking up about 20x the space per file compared to 1 copy of HEVC or AV1.
A transcode GPU like the A380 or new QSV compatible CPU is MUCH cheaper than a new good quality 12TB drive lol
Sorry for the long text, it pretty much depends on the living situation.