2nd set of leaves (after the cotyledons) turning yellow?

Is this normal? I am exactly two weeks in. Using FFOF. Havent added nutes. It is currently in a solo cup. Humidity 30%-40%. Average temps around 85-90. It's the same on the other side too.

Not normal.
FFoF is too HOT for seedlings, its getting burned.
Deff looks like N Excess with how green your leaves are.
I use FFPM for seedlings, cut with 30% perlite.
Transplant, more Perlite, Distilled water.
Also, though your plant seems not to be minding it atm, i would work on temp control for later in veg.
Shes forgiving now, but just wait... A few weeks at that high end of your previously stated temp range in veg could fry the bitch.

Should I transplant into my 7 gallon smart pot yet? People say to transplant in increments (increasing the pot size little by little) but that's just too much hassle.

I dont like to transplant in increments, I go from 1 liter to final container as soon as roots hit bottom of the 1 liter containers. I get better root development this way.

I also use fox farm Ocean from the start and never have issues with sprouts