Hello,
The Dell UD22 would only get 3 different displays on M1 as there are 2 outputs from DisplayLink technology and the other two are from DisplayPort MST.
The Dell D6000 would also get 3 different displays as there are 2 outputs from DisplayLink technology and the other one is from DisplayPort SST.
You can indeed cascade DisplayLink devices to get up to 4 on macOS. Or you can use a dock like the Targus Dock570 which use the 4-output DL-6910 chipset.
Please note that DisplayLink and Apple Sidecar share the same limit. Therefore if you use an iPad as Sidecar, you only have space left for 3 DisplayLink displays.
Here are products using DisplayLink technology we have tested
Other product exists too from the same and other manufacturers.
Provided they use DisplayLink technology, you can mix and match brands and models.
Regards,
Alban