Some ideas...
Stretch Lock Bungee Cords, if you can run the cords through the wheels. These are the shortest I could find.
Frenzy Kick Scooter. Once folded and locked into the rear brake, there will be no rotation or flopping of the front gooseneck and wheels.
Are these the ones? Are these the same size as reds and will these fit in both the A5 and A6? What's the difference between these and the reds?
Why is it a LOT better? Is this the Decathalon scooter you're talking about? Looks to be the same except for a curved handlebar.
https://www.amazon.com/Oxelo-Decathlon-Adult-Scooter-Town/dp/B07CW3H857
I think they're all the essentially same with different paint jobs/sticker. This is the cheapest one I've found with a disc brake. Came to $66 shipped with taxes for me. It's coming by UPS today, so I'll have a hand on review later.
https://www.amazon.com/dp/B08T5R4ZB1/ref=twister\_B08T67TSKP?\_encoding=UTF8&psc=1
Wow, I'm curious to find out if you're able to go 'rigid' on the rear suspension. There's a pricey dual suspension scooter sold by Micro, which I was able to return awhile back. Within the return window, the rear suspension began making squeaking noises. I believe the rear suspension on the Micro is the same as your scooter's and pretty much copied all over China. For PU scooters, I am finding that LESS is more when it comes to all the gimmicky features and extra moving parts.
I just recommended the Razor A5 DLX to someone else recently. If you look around, I'm sure you'd be able to get one for around £72. Also, consider your height - you may be too tall for it. 5'10" is probably the recommended max height for the A5 DLX. I prefer the A5 DLX over the A6, because of the wide variety of adaptable 200mm wheels as spare parts available. Razor currently doesn't even list the A6 wheels as a spare part.
Anyway, enough of the rambling. I hope you're able to make the rear suspension rigid.
scooter#1 seems promising. I'm curious about the deck width, though. It looks wide enough. Around 5" maybe? It has front suspension.
scooter#2, yes, has that disc brake. Because of its protrusion, a few people have complained about kicking the disc. As it seems promising on steep downhill braking (better than fender braking), brake carefully - the real wheel skid can be startling.
As far as suspension goes, they are gimmicky and would rather do without it. After awhile on the pavement, you may experience some embarrassing squeaking with the common dual suspension installed on PU scooters.
The A5 DLX is my base tinker PU scooter. I'm pretty sure it can be found for below £90 if you check around. Why the A5 DLX? Before modifications, the A5 DLX weighs in under 9 LBS. There's a variety of 200mm spare wheels you can swap into it, including a front AIR wheel from Micro. The A5 DLX is much quieter than the A5 LUX. Height-wise though, I only recommend the DLX to people up to 5'10". However, I've known people around 6' who had no problems riding the DLX. Happy hunting.
The A5 DLX might rattle less than the A5 LUX.
I wanted to add another. An all-purpose favorite of mine.
Xootrs are rated at a max of 800lbs. The Hudora 230 is rated at 265lbs.