For brambles, the Rapha 3/5 sleeve one is good. The shoulders/sleeves use a tougher, woven fabric than the main body, which is high-wicking, jbitted polyester. Obviously your forearms are still exposed, but only a bit of them, but your sholders and elbows and above are protected by relatively tough fabric. Come up regularly on Sport Pursuit at roughy 50-60% of RRP. The main fabric is, I think, the same as the Rapha Trail Tee and a pretty good balance of cooling and wicking.
They also do a lightweight version of the tech tee, but haven’t used that and I suspect it won’t play nicely with prickly shrubs. I wouldn’t say the straight Rapha Tech Trail Tee is that light fwiw, not heavy by any means, but nothing extraordinary, basically a nicely cut baselayer-type tee with a Rapha band on the sleeve.
Other than that, the ‘problem’ you have is that mostly LS jereys are intended for winter/shoulder season use, so tend to use a thicker fabric. The best really light summer tee I’ve found is the lightest version of Patagonia’s Capilene Cool, which is thin, but problematic when snagged on brambles – holes easily ime and, of course, is a tee rather than long sleeved.
I wonder about lightweight trail shirts, if you can cope with looking like some sort of self-consciously stylised gravelista weirdo, wouldn’t need to be cycling specific.