Shure SM58 with Focusrite Scarlett Solo 4th Gen: enough gain for a quiet voice?
Yes. At speaking level the SM58 needs about 55 dB, and the Scarlett Solo 4th Gen publishes 57, so 2 dB of gain is spare. Those are the published figures.
The Shure SM58 has enough preamp on the Focusrite Scarlett Solo 4th Gen for a normal speaking voice. You do not have to add a box in front of it.
Shure rates the SM58 at -54.5 dBV/Pa, the voltage from a loud 94 dB tone at the grille. A speaking voice is closer to 85 dB, so the preamp still has to find about 55 dB to land near −9 dBV. That −9 dBV figure is just a tracking level: high enough to use, with space before the converter clips.
Set the knob for the voice you actually record and leave a little travel for plosives, instead of pinning it because the spec sheet has a higher number. A booster on a chain that already has about 2 dB spare only adds a box. Most of those boxes also use the phantom power themselves, which will silence a condenser.