I think it's fine. You do you.
Contrary to popular belief, diet soda is completely fine in moderation (like 20 cans per day limit), assuming your teeth/guts can handle the acidity. Source: https://www.youtube.com/watch?v=oCDqqVwYMa4
Coffee typically doesn't have much or any sugar in it. It's caffeine, water, milk. It has more caffeine than soda so it feels better to take it in the morning
That's the limit of safe intake.