I'll be connecting at FRA (coming from LH F ex USA) and continuing onto BRU (LH J) 3.5 hours later.

I'd like to exit the secured area and go to worldshop. I'm also going to check an empty luggage for my shopping

What's the best way about clearing immigration? Will someone come take me to the first class lounge/terminal? I've only connected in MUC and the passport control was inside the first class lounge. Is FRA the same? Should I clear immigration at the first class lounge or do I have to go to the regular immigration. Will whoever is coming to take me (if they even come) take me to the front of immigration?