Aussie work shirts. Hi viz, breathable, sun protection and a loosish fit to allow to breathe. Long or short sleeve Polo shirt design.
https://www.workweardirect.online/colle ... eeve-3pack
I've been wearing them for years now. Bought one on first tour to Oz in 2004, mainly because their drivers are so bad and anything that helped me stand out was useful. Worn alone they are cool (I've ridden many times in the 30s C), tucked in with fleece on top or vest under, almost like a thermal top. I found them very comfortable and versatile and have bought more on subsequent visits.
Unfortunately you can't get them in the UK - only heavy cotton with stuck on reflective tape available here.