The problem with the EF is that it's pretty much unreadable at 16x16 and maybe also at 32x32. Not sure what's the scale on the top toolbar icons. Also, EF is possibly meaningless to many non-english users.
For 1920x1200 Dell 2412M (24" diagonal) monitor and Windows Display Scale = 100%, the Custom Toolbar icon sizes (derived from counting pixels in screenshots) are:
MC Size 100% ==> 16x16 pixels
MC Size 125% ==> 20x20 pixels
MC Size 150% ==> 24x24 pixels
etc.
At MC Size 100%, text in the previous icons (except for ErrorFree2) is too small to read.
At 150% everything works.
ErrorFree7.png (attached) with English text has Seqoe UI 36 point font and is readable at 100%, both greyed out and bright. Bright yellow FF0000 field indicates a warning without constricting text size, while "E F" text makes the warning explicit in English.
My current personal preference is ErrorFree6a.png (or the first posted ErrorFree.png) with the red X over a "conventional" yellow triangle intended to indicate there are no warning messages in this mode. If not 100% clear at first glance, I think the message is relatively easy to learn and remember (even though MC's actual popup messages seem to use a circled question mark on blue gradient). ErrorFree9.png (attached) is still another variation. I have tooltips turned on, which as you say also helps.
The attached "mode switch" button has a clear image at small scale but with a distinguishing yellow gradient field instead of the orange field used elsewhere in MC. Personally, I find the symbol too ambiguous here - it really leans on the tooltip.
Enhancement Request (again): Add adjustable icon size for the Custom Toolbar buttons, independent of
View > Size. For example, add a "Size" text box to the
Customize Toolbar... buttons menu. No idea if toolbar button size is currently skinnable in MC. Note: Other modern programs where toolbars play an important role, such as Inkscape and Gimp, allow toolbar icon sizes to be adjusted by the user in the UI preferences.
4/26/2024: Multiple clarifications. Add ErrorFree9.png and ErrorFree-ModeSwitch.png.
4/28/2024: FYI, I have 19 toolbar buttons with settings: Options > Tree & View > List > Show tooltips: Yes | Customize Toolbar > Text Options: No Text Labels
Win 11 Pro (64-bit) | MC 32.0.42