The Fortrez has a hardface which you won’t need if you’re just looking for a mid layer. I love the Fortrez, and it totally would work, but it will be slightly less breathable than one of the other options.

I would look at a delta lt, Kyanite or an r1.

You could go with a synthetic mid layer like the atom/proton lt, but they are slightly worse at moisture management. I’d probably go with a nano air over those for your specific use as both the atom and the proton have some trade offs in breathability for better use as a standalone.

Then again, any of the options above would work fine, it’s really just nitpicking. You could also just grab a Fortrez or a proton lt and optionally only use that one without a shell on nice days.