I think the big problem for us as players is the drop in the item to space ratio is fairly dramatic instead of gradual, especially for large houses. The problem for SE is likely item loading since that's their concern with the outdoor ward areas and interiors aren't likely to be exempt from it.
I went around to various houses I have access to and used White Marble Flooring to get an idea of the relative decorating space in each. I'm excluding the stairwell areas in the tile count since the areas are shared between floors with sharp constraints on how we can use them.
Apartment/FC room: 6 by 8 tiles for a total of 48.
Small: top floor is 6 by 6 tiles plus an additional 4 tiles by the stairs, bottom floor is 6 by 10 plus another 4 by the stairs. Total: 104 tiles.
Medium: each floor is 7.5 by 12 tiles, less 12 tiles for the stairwell on the bottom floor, 24 for the ground floor and 12 on the top floor. Total: 222 tiles.
Large: I don't have interior fixture permission in my FC's large house so I got the following by using White Partition length (which is equal to 2 tiles). Large floors are 12 by 20 less 10 tiles for the bottom floor stairs, 35 for the ground floor stairs and 80 tiles on the top floor (the huge open space in the middle). Total: 595 tiles.
So that gives us the following items per tile numbers:
FC room/apartment - 2.1
Small - 1.9
Medium - 1.4
Large - 0.7
As someone who owns a medium, I feel like the 300 item limit is reasonable even if the ratio significantly less than what a small has. A lot of my item limit goes toward building a staircase to an upstairs loft and using partitions to hide some of the wall pilasters. A slight increase to 350 items would probably be perfect in my eyes though I know there are others that would still argue for more.
But larges really need a significant increase. The decrease in items to space is too extreme compared to what a medium experiences. They should have a limit of at least 600 items.
If they made those things standard fixtures, we'd lose the ability to place them where we want just as we have no control over the placement of the awnings and chimneys. I'd love to use an awning but don't because it's not where I want it to be.
And no, not EVERY house needs/has those items. I don't use the stable or the mailbox. I kept my chocobo stabled at my apartment building until I got it to rank 20 and when I need a mailbox I go to where one is located. I'd agree that teleport to house should be standard so we don't need the Miniature Aetheryte (though I do use that for lighting effects in my yard and would miss it).