The gross salary is what it is and there are certainly (and for sure will be in the near future) opportunities that will pay more if thats what people are looking for. For me personally the package is fine and the job really something I want so I hope it'll all go well at stage 3.
Still what I find weird and unnecessary is the strange tax deal which is really unfavorable for the FO salary levels. Fully taxing and paying social security in germany would be around 300-700€ net more per months depending on marital status which is a significant sum of money over the year. I suspect things to be similar in other countries with high social security payments so I a little at loss what the purpose of this complicated setup is. It certainly would be easier to take the UK contract and tax it under the bilateral DTA. Ah well... also a thing one just has to make peace with apparently