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When I want to use 5V from a LDO as the regular main power supply for my circuit and only want to switch to 5V USB from the USB connector when no external 5V from the LDO are recognised, I could do that with the following circuit I guess:

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But I would need another LDO to get back to constant 5V since the schottky diodes reduce the output volatage?

Is there any way of reducing the forward voltage of the Schottky diodes, so that I dont need another LDO for such a simple task? Or is there a simpler way of prioritising V_ext over V_usb?

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Yes you can use a power multiplexer made exactly for such applications to avoid the diode losses incurred. They come in many variants but the one you're looking for for your application should be widely available in may variants. Look for "power multiplexers" for power-supply switching.

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