At some point increasing tyre pressure = more grip, (somewhat counter intuitive) so may have added to the spoke issue, but also more rear grip would make the understeer issue worse.

Ok, the whole tyre thing is getting more complicated... in theory
https://www.f1technical.net/forum/viewtopic.php?t=12762
Will be glad to try it out on the track.

I’m only surprised in as much as I suspect that 90% still follows simple engineering principles and you only need to get into the details and nuances to extract the last 10% in more extreme performance than we hope for from cyclekarts.

I’ll mix in some experimental evidence and see what I conclude.

I realised that I didn't share my new brake set-up.  I went through a design process with the aim of having enough braking force to lock the rear wheels with a target foot travel and pressure.  So far I'm very pleased with the result.

0.7" Wildwood master cylinder from Rallydesign.co.uk http://www.rallydesign.co.uk/product_info.php?products_id=4052 £32
Front calliper from a 2007-2012 Suzuki GSF 650 Bandit £20 from a breaker on eBay.  The one I got was in good condition but you might need to replace seals on 2nd hand units.
Standard Gemini brake disc

The brake calliper has 2x 27mm pots and 2x 33m pots (CSA = 28.6 sq cm)

On no account should anyone be foolhardy enough to take any notice of what I've used.

Well I finally got around to finishing the tacho and it works.  It took a bit of hacking. I'm sure there's an easier way to get a low-cost working tacho for a single cylinder engine so if you know one please do share.

Here's what I did:
I bought a cheap "universal" tacho from eBay - no documentation and the cables were +12V (black) Ground (green) Signal (yellow), plus others for the backlight.
This takes a signal from the low voltage side of the ignition which helpfully is also the feed for the GX200 kill switch.
When I hooked it up, the tacho worked but because its designed to read up to 13,000 rpm I wanted to change the full-sweep range to 4500 rpm.
The circuit is based on a BAK225 chip.  I couldn't find any documentation for the chip so it "just" took a bit of experimentation to find the right resistor values to get it about right.  You don't have to change the variable resistor, but mine broke when I was adjusting it
I've got an optical handheld tacho but it doesn't give a very reliable reading so I've calibrated the new tacho as best I can.
The dial face is waterproof sticky back vinyl for inkjet printers stuck to aluminium and the needle is a clock makers part also from eBay.
 
I hope it doesn't shake to bits when I fit it to the cyclekart.

Im wondering if changing the reset cap from 104 to 224 might also cut the revs full scale deflection in half, due too slower integration time, without changing the resistors. Or just solder another 104 cap in parallel with the original one.

 Bit of milling, a bit of welding, some knife handle making YouTubes and the Bugatti has a gear shift lever.   Not sure what it’s going to shift, but should make for a nice bit of bling.