First, I'll have a dipole in the attic.

Second, I realize the Comet CHA-250b is a compromise antenna, and their "no radial" thing only changes power to heat.  It was free.

I want to put it up in Flower Mound, TX, in an established neighborhood with trees.  I don't want to burn the house down, or cause a lightning strike.

I would LOVE it on top of the house to get some height, but that seems to be courting disaster.  I can always put up a 10-20 ft tall "tower" thru one of the trees in the backyard, but I'd need to run 150' of lmr400 to get to the shack.  (I DO have a remote tuner).

What are some things I can do to mitigate lightning issues?

A 20 foot antenna ground mounted with radials will be more effective than a roof mount without by a wide margin.

Plus it's a smaller lightning target.

I'd skip the roof mount and just move it away from the house, and of course disconnect when not in use.
With a remote tuner you'll be in great shape. Before I switched to my 43' antenna I was using a 16' antenna
with a loading coil (Buddie stick with their military whips.) It worked surprisingly well, and even a modest radial
field at 0.1WL (basically eight 16-foot radials) was enough to make is passabily efficient.