Trains not running deliveries
1) Check provide and request threshold, if none is set global setting is used
2) Check if train size is plausible between requester and provider
3) Check if Network ID is plausible between requester, provider and depot

Changing train composition makes trains switch to manual mode
Workaround for [this behavior] (https://forums.factorio.com/viewtopic.php?f=23&t=38850). After you're done rearranging trains send them back to the depot in automatic mode to register with LTN again.

Unloading fluids with circuit controlled pumps leaves residue
Support [round up fluids reading train inventory] (https://forums.factorio.com/viewtopic.php?f=6&t=49274)
Meanwhile don't use Stop output fluid values to control pumps for unloading. Use [Inventory Sensor] (https://mods.factorio.com/mods/Optera/Inventory%20Sensor) instead to have fluids always rounded up.

Trains longer than 31 carriages don't display train composition signals
Nothing I can do about that, the circuit network is signed int and can only display 31 bits.

LTN uses a lot of update time in megabases
Increase update frequency in mod settings. At an update frequency of 5 I'm running a 2k spm base with over 500 stations with LTN taking up only 0.05ms update time.