Is my 4K projector's dull picture normal?

Other times the center of attention is intended to be drawn to the presenter, so the projector is much more suitable than individual screens would be.

There is a difference between business projectors and entertainment models. Either way to get the most out of it I would say you definitely need a high-performance dedicated projector screen, maybe check with the pros who rent the AV gear to conferences and get their advice on the ones they think work best. Without them actually trying to sell you products it should be possible to get some honest evaluation under non-ideal conditions.

A projector cannot take light away. So unless your room is truly dark (no candles even), you cannot expect black. “Dim” is not nearly enough.

Looking to do outdoor video graffiti and VJing.

You appear to be thinking in terms of resolution, where you'll get better details/sharper focus the higher you go.

For projectors, I've learned to move up in lumens. 6-7k minimum.

Keeping in mind: the higher the luminous flux, the more expensive.

-Better brightness, contrast, color, etc.

-Screen border edge contrast with surrounding wall gives better ‘outlining’ to the picture, helping focus.