Just joining this thread.
Looking at the tank gouge, it is hard for me to see exactly how deep it is. Maybe you could find a steel scale, or maybe a set of wire feeler gauges (Spark gap gauges) to determine how deep it actually is. It might be not be as deep as it seems because the plastic that was displaced to the sides makes it LOOK deeper.
I have repaired various poly tanks with a large soldering iron and some sacrificial material from elsewhere on the tank, or from another item altogether. Basically, you scrape the surface clean, heat the wound until it turns translucent, melt some filler material into and onto the area and build it up.
In your case, it is not all the way through, so if you did decide to do some plastic welding, that's all you'd be doing; filling the groove and building it up.
Here is a video that I did. (I'm sure there are many more on YouTube)
https://youtu.be/GiBrIlpGSvU