I remember using XDMCP ~20 years ago for this (underpowered machine as thin-client to beefy server, with graphical login). Does LTSP have additional features or better performance?
LTSP Clients boot with PXE and get the root filesystem via NFS, then it uses XDMCP login screen.
AFAIK, LTSP 5 uses LDM instead of XDCMP.