Your poppet looks OK to me, but without seeing it first-hand, it's only a guess.
Make sure you are using a fresh O-ring on both the dip/gas tubes, and another fresh one on top of the post. Tighten the post reasonably tight but do not overtighten.
"Thinning" at 4 days? I assume that it's near the end of fermentation and not carbonated. Before determining if it's thin or not, you need to carbonate it at the desired volume.
If it's thin after carbonation, you need to review your mash temperature and increase the temp to increase body.
in THIS case Topo Chico is sparkling water. But Coca-Cola bought them a while back and is selling hard seltzer under that name now. The fact that the bottle says 0 calories leads me to believe it's sparkling water and not hard seltzer.
I'm by no means an expert, but I had decent results by using cream of tartar.
The final temperature determines the "crack level". The level of darkness was determined, in my case, by the number of water additions, so going from to cooler, to hot, repeatedly (be careful as sugar syrup can...
The last thing that should come in contact with the items that touch your beer/wort should be sanitized.
From boiling to fermenter, the only thing I sanitize is the tip of the hose going into the fermenter (and the fermenters themselvers obviously).
When I transfer from primary to secondary...