Ah, the ol' Westwood-style RTS. Tiberian Sun was my favorite! And the Rock Raiders should be glad their Energy Crystals aren't Tiberium crystals
I like moddable GUIs. Unfortunately, GUIs are one of the least moddable parts of most games. Probably because they mix data (usually moddable) with code (usually not moddable, unless there's scripts), and it takes some effort to get that separated out.
Idk if modern Unity has this too, but some engines like Godot allow loading GUIs from data files containing the Button/Image/TextBox/whatever objects with their positions, sizes, event function names, and other variables. That's just a lot more work than implementing the GUI inside the engine. Maybe not worth it for a simple game?