So this is not about unupgraded rooms not matching, but that even upgraded ones do not fit in?

Hmm, I guess the option to use a lower-tier road instead would not cause any problems.
It would not be an easy fix, especially with how to put that in the UI.

For example marketplaces and other "rooms" that are not really rooms but just structures outside have some kinda tile under them. I don't get how this tile is determined other than that it goes up with the upgrade level.

For example I still get forest plaza tiles under some stuff that has different plaza types around them. This is a bit annoying because it looks out of place.

In my opinion they should analyse all the tiles that border them, and then put whatever is on those tiles, unless they are upgraded and there is an empty tile, then they might pick a fancier one. But if there's an actual road tile, they should pick that one regardless of upgrade level in my opinion. Probably you misunderstood me because that will not take any UI upgrade.

If there's no road tile around them then it can be a dirt road. If there's a wall or other room I would suggest to disregard that particular tile from the calculation.

You could also consider making it manually assignable by the player, but that would

This would need a UI component because not everyone will want this.
Personally, I like to be able to see the upgrade level at a glance, and also like it when the fancier ones break up the efficient road I put everywhere.

Ok fair enough. It's nothing to the gameplay so not really a priority I would say. But some player control over the background tiles of outdoor buildings would be good. It can be limited by the upgrade level still (e.g. no plaza on lvl 0 building)