This may apply to a few of you guys:
I could not find the HD Textures option in the Video options either. But then I remembered that I had previously set the file gamerprofile.xml to Read Only under C:\Users\<username>\Documents\My Games\Far Cry 5\.
So I removed the Read Only attribute, opened the file and set the following:
HDTexturesOptionSeen=1
HDTexturesEnabled=1
Then I set the file to Read Only again. And now the HD Textures option is available under Options > Video > Quality.
I have to say though that the difference is pretty minimal. And it only seemed to increase the already present amount of small stutters with v-sync enabled. So I decided in the end to not use the HD textures.