Is an upgrade from Lumix G3 viable for a noob photograhper?

You are in almost exctly the same position that I was in 6 months ago, even down to the same two lenses excpt that I had converters for both Nikon F mount and Minolta MD legacies also. My former camera was the G1.

I bought a nearly new G5 for £250 and the upgrade delighted me, the new sensor was a major advance.  Like you I had no interest in video but the G5 changed that in pretty short order once I saw the result of a couple casual try-outs.

In all respects the G5 meet all my quite exacting demands and none of my many photo colleagues now decries 4/3rds it as toy cameras. Like all 4/3 cameras you must shoot in RAW to realize the full potential.

So my (very biased) advice is to go for a G5 (eBay £250) and use the saved money for the best quality lenses that you can afford.  The first step must be to swap your 14-42 for a 14-45mm, the uplift in image quality will really surprise you. Keep the 45-200mm (but avoid using it above 180mm as the image falls apart when fully extened (you will get much better results cropping from 180mm images).

If you like big skies and landscapes and your budget runs to it, add a Panasonic 7-14mm (around £500 but worth every penny) or an Olympus 9-18mm (almost as good and it can take filters too).

As to macro, the early series Tamron SP90 beats everything else including any Nikon or Canon costing under £1,000 - suitable mount adaptors are dirt cheap.  You should find a Tamron SP90 for around £150. It needs a bit of effort to get the best out of it but the results can be stunning. If you can't get on with it there are dozens of people on eBay who will buy it back from you for what you paid for it, so it's effectively available on free trial.

Alternatively, see if you can pick up an early Nikon, MicroNikkor AI 55mm macro. This professional quality lens can be found for much the same money as the Tamron but its shorter focal length can be a bit of a disadvantage when photographing insects and flowers