Thank you for the hint. I tried but the pricing is a bit disappointing. I started with an LTE modem mini-PCIe card that contains a modem module and some simple components (coils, a voltage regulator, U.FL connector).
The offer from JLC for 1500 pieces (without the modem module) was 2000€ + Tax + Shipping, so way less than 3000€.
The comparable best offer from Eurocircuits was 11000€, so a factor of 4.
JLC can even be optimized further (e.g. panelizing, daves another 500€).
I hoped that I can produce here in europe with a maximum price factor of 2 in comparison to JLC which seems not be possible. It's cheaper for us to employ someone who manages JLC than producing in europe - especially when going to mass production.