In the older I get the better I was column...

I road across the Mohave in 115+ with long sleeve wool jersey.

If the base layer wicks well - wool (I hear Yak wool) it will be cooler.
It is also better for crashing, if you do that kind of thing.

Best thing I've used to date on a hot ride is an absolutely skin-tight Pactimo SS jersey with nothing under it, shorts (not bibs), white sunsleeves, and a white skull cap under my helmet, 'cause I'm mostly bald.

I'll occasionally wear a long sleeve Champion t-shirt in their thin, lightweight wicking fabric, white or hi-viz yellow or orange, under my cycling jersey. Works well on hot sunny days that aren't too humid.

The Champion tees use a very thin material that isn't substantial enough to support pockets, so it's significantly thinner and lighter than most cycling jerseys. Soft and comfortable, breathes well. Doesn't cost much. These aren't form fitting, however. Champion also makes compression tees, long and short sleeve. Haven't tried 'em yet.

To me it's more comfortable and less hassle than using sun screen. I still use sunscreen on my face and neck, and exposed areas on my legs. But it feels sticky and my legs in particular get grimy after a long summer ride. The long sleeve tees are a good compromise for sun protection and comfort.

I really like the Pearl Izumi Thermal baselayers year round, except for the most humid summer days. And I've worn it under dress shirts for summer visits to museums, theater, etc. But they're pricey and I have only a sleeveless baselayer. I might try a Champion compression long sleeve base layer to compare with the Pearl Izumi.

I feel that mesh base layers keep me cooler but have discovered a more important benefit in that it intimidates other riders because it just looks pro.

Unzip the jersey half way, let that white mesh show and nobody dares an attack.

In fact, that's the only downside. As soon as riders in the group see my summer mesh base layer and feel the aura of power it projects they immediately fall in line on my rear wheel. It is difficult to get anyone to take a pull.

I tell you what, nothing says you are ready to drope the hamer and lay down 400 watts like a summer mesh base layer.