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.