I have a substantial interest in trams, though my model-making in a couple of scales is suspended at the moment thanks to circumstances. In the context of the question, I had also acquired a started Occre 1/24th kit of a standard gauge tram, and plan is to improve fidelity and make it more robust and operative, cheating slightly by using a scratch-built truck, G3 wheelsets and purchased track for a garden loop.
As for parts, I know in the past there were aspirations by at least one small supplier for G3 parts, ready-made trucks etc., but not sure whether these came to fruition. For tramway modelling, the traditional organisation has been the Tramway and Light Railway Society [TLRS], which has a little modelling information on its web pages: Tramway Information
There are area groups in the UK, some of which have strong interest in G3 or similar. TLRS run a journal with six issues per year which has quite a good coverage of prototype, models and layouts and a good section on new model products in all scales.
Interesting to hear of your aspirations for G3 standards and it sounds like you've approached the issues in a really thoughtful way. I hope you achieve progress. I've noticed that much of the high fidelity tramway modelling has come from those with appreciation or experience of present-day railway modelling both in scaling and in quality. By coincidence today in my travels, I recognised someone in the distance who is a fine exponent of P4 rail modelling, and I was reminded of their diorama of a London street tramway using P4 standards, yet incorporating a conventional 00 kit of a Feltham car with the necessary finesse added.
Like all modelling, both the builder and beholder are free to choose their preferences but the compromises with tram models can lead to poor aesthetics. Tramway layouts, particularly in the smaller scales, even are more liable to downward viewing. A visually realistic railhead dimension, typically narrower than 'heavy rail' in tramway usage, is more important than the code of the rail if street track is used. In 4mm scale, the widespread use of 16.5mm gauge to represent a standard gauge prototype tramway really brings out the visual flaws of that compromise. Model tram wheels also benefit from being narrower in tread than a railway equivalent.
G scale (whatever that is!!) is to 1:22.5 running on 45mm track