OP; the Specialized/Roubaix stem you have along with the 20mm of spacers gives you a lot of “flexibility” as you get more flexible, strong and comfortable sustaining a good aero position. Flipping the stem in combination with the supplied +/ - 8, 16 degree shim along with the std (2) 5mm and (1) 10mm spacers gives you multiple options from the start. You can also drop your degrees gradually over time by picking up from the Spec fitters toolbox +/-10, 14 and/or +/-12 shims.

Aero in a comfortable position of power is king. Getting the body to drape properly over the bike into a well balanced reach to bar comes with the body maturing. Your thoughts to go incrementally are wise. Test over some miles on the hoods and in the drops and then make small adjustments.

I also assume your saddle position is optimal for you and keep that big melon that we all share in our helmets “eyes forward” and resist the urge to slam it crown first into wind. THAT’s adds to an already big drag. Work those neck muscles as you drop down. Good riding.