Qt meets all your requirements; everything (GUI code and business logic) can be implemented in moderate C++ (C++ 98 or higher, even "C with classes" style is ok

Qt meets all your requirements; everything (GUI code and business logic) can be implemented in moderate C++ (C++ 98 or higher, even "C with classes" style is ok). RAM usage is generally low (< 10 MB for most of my apps). You just need the Qt base package (Core, GUI, Widgets), and version 5 is still good enough. If you prefer C over C++ https://nappgui.com https://nappgui.com is yet another good library.