[Bob Bengal]

I have a 3/8" Freud spiral upcut bit coming soon

A 3/8" will work, I'd use a 1/4" up cut spiral. My reasons: less work for the router. Put the wear on a less expensive bit. Less saw dust.

I plan to plunge this, but if it's easier, I could use the other base and twist the motor into the cut.

I vote for the plunge. I would never adjust the height of a fixed base while the router is running.