Your keyboard amp has eq already, I assume. If you haven’t, I would adjust that, prioritizing the vocal quality over other instruments if all channels share the same eq. One benefit of a “real” mixer/PA is that each channel gets its own EQ. Reverb is the second most important effect, I’d say. And delay is optional, depending on the genre.

If your mic sucks, that also might contribute to “sounding bad.” Does everyone agree the singer sounds bad on your system? If you aren’t used to hearing your own voice objectively, you might be thinking it sounds worse than it actually does.

Look into mic technique. The basic idea is you have to actually sing directly into the mic (NOT over the top, and don’t hold it like a comic facing straight upwards; it should point into your mouth), making sure you are following the polar pattern of the mic.

With a dynamic mic, there is something called the “proximity effect.” The closer the mic is to the singer’s mouth, the warmer the sound is gonna be, while farther away makes the sound thinner. But you have to pull the mic an inch or three away when you sing especially loudly or high pitch or you will break everyone’s eardrums.

A good baseline distance is a finger’s width or two between your mouth and the mic. If you sing very loud in general, 3 fingers might be appropriate.

And during mic check, don’t just talk. Actually sing like you’re actually planning to sing. Seems obvious but many singers just say check 1 2 but that’s usually quieter and less melodious than singing.