I've been testing 2 basic systems right now.
One featuring a merino next to skin layer and one featuring a poly next to skin.
Specifically I've been putting my sitka core lw short sleeve up against a kuiu merino 125 t.
Different people have different opinions and preferences when it comes to comparing natural fibers (cotton or merino wool) to polyester.
The thing about natural fibers is that they will initially breathe better. But that is if they do not get completely saturated with moisture and sweat from strenuous physical exertion. If you will be sweating for a long duration of time, then the polyester will absorb much less moisture and remain drier. It's not fun to wear natural fiber material that is drenched in sweat and wet.
Some people can't stand the feel of polyester against their skin. Though that can still be better than moist or wet natural fiber material.
Namely when I have them covered by a mid layer they act differently. When I wear the kuiu merino 125 under a fleece it actually seems to not dry as well as the sitka. When I cover the merino with more merino in the form of a kuiu 145 hoody it seems to stay more dry as if it pulls the moisture into the 2nd layer of merino. It also seems when I wear the 145 over a synthetic base layer it pulls moisture into that outer layer more as well.
That's no surprise at all. The fleece layer (being made of synthetic polyester) probably does not breath as well as the merino. That means, even though it will let air through it, it will still not carry away moisture from the inside to the outside as fast as a natural material like merino will. This causes you to sweat more, and will make the base layer more wet if it is natural.
So it seems like in your situation it may not be advisable to wear a natural layer under a synthetic layer. Either they will both have to be natural layers, or both synthetic.
(I mean for you personally. It may be a different situation for other people)
What you decide to wear may also depend on the temperature, and the weather.
It might be better for you to wear a very lightweight merino wool sweater, or if the temperature is not too cold, maybe a loose long sleeve cotton shirt or zip hoodie.