I currently own a 27” LG monitor and a U3818DW, but would like to replace the LG with a Dell for the Display Manager and daisy chain. My question is about affects in resolution by running my Lenovo T480 -> TB3/USB-C -> a lower resolution monitor like the U2520D’s 2560X1440 and then DP/mDP -> a higher resolution 3840X1600. Would the primary monitor cap the resolution to its native resolution, or would it pass through as much as possible?
That configuration was never tested by us, so unknown. We never even thought about such a combination. The U3818DW User's Guide doesn't show any multiple monitor configurations. The U2520D User's Guide page 34 has this stipulation = The maximum number of supported monitors via MST is subjected to the bandwidth of the USB-C source.
IF the Lenovo Lenovo T480 TB3/USB-C can provide the necessary bandwidth, then possible. You will need to post on the Lenovo Forum and ask the users there if the Lenovo T480 TB3/USB-C could actually push the U2520D 2560X1440 60Hz plus the U3818DW 4K 3840x1600 60Hz.
FWIW, I have a Lenovo C940 daisy chained via TBT3 to a U2520D and a U4320Q. Both monitors run at native resolution: 2560x1440 and 3840x2160.
Not exactly the same config as your T480 which is, I think, an 8G Intel w. discrete TBT3 controller where the C940 is 10G w. integrated TBT3, but I don't know if that changes the total bandwidth Chris asked about.