It seems I don't need scanadf. I can scan from ADF using scanimage:
sudo scanimage --source "ADF duplex"
That might be a specific option for the HP 8200 series.
Unfortunately, I always get a segmentation fault for the second page.
Your specific error is because the Raspberry Pi kernel header package doesn't follow the standard naming scheme. The displaylink-debian script needs to be edited to change all instances of linux-headers-$(uname -r) to raspberrypi-kernel-headers so that the Pi's kernel headers are installed properly.
If the script still doesn't work, there may be other ...
I have a Pi 4 Model B/4GB serving a small office as a router/firewall/gateway with dnsmasq (DHCP+DNS), dnscrypt-proxy, chrony (NTP), and a few other odds and ends. It's fairly busy: up to 20 users at a time, lots of file uploads, three main VLANs (plus a management VLAN). There's minimal internal routing for the time being.
I haven't really stress-tested ...