Say, for example, I want to create a "Starfire Ruby" specifically that doesn't fall under the 5,000 gp value, because it's been badly damaged, and worth maybe 200 gp...

I don't see a way to create that, other than create a (Magic) Item and just give it that info. But then it asks for the type (Ring, Scroll, etc) and doesn't have just "Item" 

EDIT - Along those same lines, creating a magic quiver (say like https://www.dndbeyond.com/magic-items/efficient-quiver ) - I don't see a way to do that either. Magic Item forces the same thing - select a weapon. I don't see an option to make it a "Container"

Would be cool to have that feature - or if it's there, I am oblivious to it. :)

Quivers, bags of holding, etc are all Wondrous Items, so just use that. there is no "Container" item type. Any magic item that isn't a weapon or armor is a Wondrous Item. that includes things like gems, quivers, bags of holding, soarsleds (not actually in official content), capes, pouches, whatever.

and homebrewing mundane items is on the roadmap as part of the homebrew revamp planned in the future.

yeah, i just use the magic item section to create things like poisons, i homebrew a lot of poison

For something like the ruby, where all you really want to change is the value or superficial description, you can just change the value of the item. I do this with diamonds all the time for my cleric -- the description in the sidebar still says 5,000 gp, but the edited cost is now 300, so that's what's immediately visible in the equipment section.