In short:
Replace, merge, or add the "Size on Disk" filter on steam with a "Space Required" filter instead and add a "Unready to Play" button right next to the "Ready to Play" button to combine with these filters.
OR... Second option is just to have the "Size on Disk" filter show the "Space Required" games with the "Unready To Play" button enabled.

Explanation:
I love how steam lets you use the "Size on Disk" filter to help you find which games are taking up the most space and easily access most of your AAA games or AA titles but this only works for games that are actually installed and really only largely help you manage your disk space (something you can already do better if you go into settings) or look at the games you don't have installed.

But since I have an exceptionally large amount of very small in size indie/old titles. I often find myself having to sort through them all to find my games whenever I have a lot of extra space or when I'm looking for some bigger titles to preload on my device to play later. I've pretty much resorted to using the "Metacritic Score" filter because we all know they pretty much only review AAA or AA games unless the indie titles are exceptional or popular (60% of my library doesn't even have these scores).

How do we solve this?:
You could make this better by replacing the "Size on Disk" filter with "Space Required" and just make it so you can see the "Size on Disk" option if you have "Ready To Play" enabled while using the "Space Required" filter. This will not only help you find smaller, easier to run indie titles, but also will help manage your disk space for your SteamDeck's limited storage(something i struggle with constantly). This will also allow you to find your bigger AAA/AA titles on your PC without having to sort through a giant backlog of weird indie games and old games that don't take a lot of space which is something you would want for the SteamDeck or any SteamOS handheld.

So If you replace it, the only thing you are missing out on is the ability to see all your uninstalled games with "Ready To Play" off (which is something I assume maybe not even a lot of people know you can do with the "Size on Disk" filter in the first place). So what you could do to make this even better is just add an "Unready To Play" button next to the "Ready To Play" button. Combining these will not only let you see all your uninstalled games but have them neatly and perfectly organized by the space they take or the space required.

Instead how you could also make it even simpler is to just have the "Size on Disk" and make it show "Space Required" instead if you have "Unready to Play" button enabled.

This is something that would be so amazing to see!