Hi Brendan, I had the exact same problems you have been having like the terrain glitch, and the EXACT same gigs not working. I searched and searched and no one had the problems I was having.. until I found your post.
I was obsessed with finding a fix and finally I found something that made the problems go away.
I don't know what it was honestly.. I remember everything I did so I'll post them below.
-First I went into my directory and deleted EVERYTHING except for the game launcher.
-After that I went into my bin and deleted everything from my bin, etc.
-I then updated my Chipset LPC Controller ( No idea if this is even needed ) and used the windows updater (NOTE I DID NOT DOWNLOAD VERSION 21H1)
-I restarted my computer then verified the integrity of my files through steam.
Of course there was stuff missing I deleted them and emptied my bin.. I let it find and re download the stuff i was missing, then loaded up the game not expecting much.
I was shocked when i scanned trevors body and johnny popped up, I then got the prompt to carry his body to the car... YAY.
i then tried the gig SOS: merc needed and got the phone call... I haven't tried the others but so far I am shocked it's working.
I hope this works for you as well.. you were the only one with the same issues as me.
EDIT: terrain texture bug is fixed! its not my graphics card whew !