How do I get a good screenshot of my map?

When I launch the map from mod tools is there a way to activate noclip/flying and disable the UI?
Or is there a way to use the in-game theater mode?

there are a few commands you can use

"\ui_enabled 0" is one that disables most of the hud
"\cg_draw2d 0" disables the crosshairs and other shenanigans
"\cg_drawgun 0" removes the weapons if you want to do that
"\noclip" is how you activate noclip

If you want things to slow down use "\timescale x" and replace the x with whatever number, 1 is the default speed, anything lower slows time down, and anything higher speeds it up

Or you can fly around with an xbox controller in radiant with Game view set (F9). To take still shots you can actually do that within radiant go to File>Screenshot (or something along those lines), pick HDR for high definition (High-dynamic-range) image and select the quality options to suit.