Much appreciated, thanks @rkoswald & @pbase! Though, I’m thinking I should’ve asked this in a different thread.
Current location is Brisbane (27°S). Earlier this year I was living at Hobart (43°S) where it’s about 10°C cooler & also drier. Massive difference! Actually I grew up further north than here, just somehow never got used to the heat & humidity. I moved to Hobart seven years ago as a climate refugee but I’m back because of family.
RK that group ride sounds brutal! How was the rest of your day after that?
Good point about difference in ability between sweat & plain water to evaporate. The track where I do workouts has water taps & I’d got in the habit of dousing myself between intervals. A few weeks ago when attempting Atitlan I realised it did not seem to provide sufficient relief anymore. Maybe I need to start before I get sweaty… as in, drench myself when leaving the house. Haven’t done a sweat test but I’ve settled on the maximum salt content I could stomach, which is about 3g of salt in 800mL bottles, so Na=1.4g/L (which I’ve found in the middle of an interval workout, now tastes better to me than plain water) & also supplement with magnesium at meal times.
Consuming cold water.
It came to my attention several years ago that ingesting copious amounts of very cold or hot liquids could change the temperature in the stomach, & impact the chemical reactions that need to take place, in this case breaking down sugars, so I’ve been reluctant to do that. But there was no science to back that up. Maybe if it means I can only take up 60g/h instead of 100g/h, the workout is only 1½h, & I’m already carbed up then it’s not so big a problem. Maybe a Dr Kyle question.
Ice socks & more water blocks could be a possibility. I ride with a small top tube bag. Not keen though on leaving a backpack unattended at the track because it’s open for all & sundry to wander past & through, but I am in the market for a frame bag. Stashers are insulated & could be worth a shot, either with ice or just a cold water bladder. I love how their capacity is measured in beer cans!
Don’t think I’d want to drink something bubbly or fizzy that’s been carted around in an unsuspended frame bag though. 
Kit is Neopro, jerseys are super thin. I don’t wear base layers. Always wearing light &/or brightly colored kit. I’m already a system-bucker, looking like an aspiring triathlete, because I wear ankle socks & ride with aero bars. So I don’t mind going further down that road.
pbase I agree on sun sleeves… anything on my skin traps more heat than the heat in the radiation it reflects, unless as you say, it’s wet. 
Sunrise today was 4:48 which makes for a very early start! I generally get on the bike around 5. But I agree: pre-dawn & that first hour after are the best.
Thanks again for all the tips. I’ll start with the lowest hanging fruit & try to keep doing interval work. Trad base until autumn is not ideal because of the timing of a race, but if it ends up being the only thing I can do, aerobic fitness is never a bad thing!