I like the way Old-School Essentials aims for brevity and usability, but it sometimes is a little too terse. I don’t think it would work as well if it weren’t restating B/X. Notably, Dolmenwood is more verbose than OSE. I agree with
@Malmuria about respecting the spread, but it’s not a silver bullet.
Worlds Without Number is designed around the spread, but it’s also presented as dense walls of text. I found it difficult to find things when I ran WWN.
I think my idea is terse but conversational. Build up concepts and make it easy to look them up again later. It seems like if you do that well, it shouldn’t be necessary to produce separate reference sheets. For custom rules elements (such as abilities, spells, etc), I lean towards using structured elements to present them. You can see this in 4e with powers and PF2 with feats as well as other games like
Konosuba TRPG and
Goblin Slayer TRPG.
One non-TTRPG inspiration might be board games, especially those that incorporate cards. They have a standard format that helps you parse the rules elements they contain quickly. You can tell at a glance what they cost to play, what they do, and if they have any requirements. It should be just as easy to understand what a spell or feat or power or whatever does as it is to understand, e.g., what a project card does in
Terraforming Mars.