New 1x10 project advice?

Not sure why you are worrying about a chain guard on a single chainring setup. In the good old days of "5 speed bikes", we didn't worry about the chain coming off, as this usually happened during a during a front shift (which you won't have). The only exception was Cyclocross racing where a chain guard was used on both sides of the chainring.

Except with a 10 speed cassette there is a greater range of movement leading to more extreme chain angles at either end if the cassette so I higher chance of it coming off. I'd day you'd probably get a lot of chain noise in the highest and lowest gears. You could live with it but it'd wear out quicker.