First, you need to understand that Lightning and USB-C are two entirely different interfaces - electrically, physically and signalling protocols. Interface conversion requires more than simply “adapting” the differing physical connections.
The commercially available Lightning to USB-C adapters are largely intended to provide basic data and charging services - whereas your existing proprietary Lightning Adapters go beyond the basic capabilities of the adapters.
In the main, you are unlikely to find functionally capable USB-C adapters for your Lightning accessories. The best advice, with you having chosen an iPad with a USB-C port, is to accept that your new iPad is not designed to interface with Lightning devices. You would be better investing your time and effort in capitalising on your significantly more flexible USB-C connectivity.
For example, a suitable USB-C hub device can simultaneously provide connection to multiple devices. As an illustrative example, this Anker USB-C hub supports simultaneous USB-PD pass-through charging, USB Type-A and USB-C connections, HDMI, Ethernet etc.
https://www.anker.com/ca/products/variant/powerexpand-6in1-usbc-pd-ethernet-hub/A83650A1