ImRAD is a GUI builder for the ImGui library

Same in Delphi / Lazarus. And the language was / is way more powerful.

I can't fault Pascal. It may not have every module ever like Python, it's syntax may feel archaic but the satisfaction and feel of producing a program with is fuzzy feeling great.

I enjoy making my own things, especially wheels.

There is a 3rd party "Python-for-Lazarus" bridge[0] though for simple stuff you can use the python library directly:

    program foo;
    {$MODE OBJFPC}
    uses CTypes;
    const PyLib = 'python3.11';
    procedure Py_Initialize; cdecl; external PyLib;
    function PyRun_SimpleString(Command: PChar): CInt; cdecl; external PyLib;
    begin
      Py_Initialize;
      PyRun_SimpleString(
        'from tkinter import *'#13 +
        'window = Tk()'#13 +
        'window.title("Hello, world!")'#13 +
        'button = Button(window, text="Clicky")'#13 +
        'button.grid()'#13 +
        'window.mainloop()');
    end.

Also depending on what you want to do chances are there is already a Free Pascal library or existing bindings to a library written in another language. If not, if the library has a C header file you can use the h2pas utility that comes with FPC to make your own (though the generated code does need some modifications to work more often than not).

Of course all the above do assume you have knowledge of both Free Pascal and whatever you want to interface with, but at least as far as being able to access existing libraries is concerned, it should be possible - just with some hoops (and at that point it depends on how much you value whatever Lazarus provides over the effort involved in whatever library you want to use).

EDIT: and truth be told IMO the biggest roadblock with Lazarus isn't so much the language but that it is just not easy to learn most of that stuff (the core language and runtime library is decently documented but anything outside of that can be a hit and miss) and you basically need an almost arrogant attitude of "this is just dumb code, not magic, if it is technically possible i can figure out myself how to do it" :-P.

[0] https://github.com/Alexey-T/Python-for-Lazarus

Free Pascal is not as fast as Delphi 2 (especially if the target OS needs to use a native linker like GNU ld), for some decently sized projects it can take several seconds (or even a minute for multimillion lines of code) for a full build, though partial builds tend to be almost instant - at least in hardware released within the last 10 years (and SSDs help, like everywhere else). Of course both the language and the libraries (and Lazarus itself as an IDE and its own framework) do way more than Delphi 2, which itself was also more advanced than VB6.

I do believe that the provided functionality scales well with its requirements.

I'm not sure about a 400MHz P3, but i have tried a git build of Lazarus from last year on a late 2003 Athlon64 PC running Windows 7 and was very comfortable. A 400MHz P3 would be faster than an original Raspberry Pi, so i'd expect that if nothing else to be usable though might not be as comfortable as VB6 (or Delphi 2 for that matter). I do have a (IIRC) 800MHz P3 around for retro games, at some point i might try to install some lightweight Linux for fun and try to see how it performs there.

more generally, guis that don't need to run constantly at xx fps don't have much reason to not be written in html5

So, yeah, there seems to be some desire for that kind of thing out in the wild these days.