MacOS
LRTimelapse 6.0.0

Source Resolution cannot be saved in Preset ... automatically selects 1080p

Hmm, works here. I've created a new preset with source resolution, named it "Source", it appeared in the table. Switching back and forth correctly set the "Source" resolution.
Then I clicked on "Set as Default settings" bottom left of the dialog, to take over everything.
Restarted LRTimelapse and the preset is still there and correctly sets source resolution.
Could you please double check?

I sent you a mail with a video / already tried to reset all settings. Still the same.
Check on two Macs with fresh accounts and fresh LR Timelapse Installation.

On one of the two I now successfully have saved as "source" ... but I tried severals combinations and restarts and don't know why its working now. The other is still saving every Source Preset as 1080

It seems to work, if an intermediary sequence is loaded, but not, if you have a "fresh" Renderdialog, without any sequence loaded.
This will be fixed in the next update.

Until then you can help yourself by creating the preset in a different resolution and then editing the LRTimelapse.config in a text editor (LRT must be closed). The location of the config can be found at the top of the log (info menu / show log).
In the config find the preset that you want to set to "Source" and simply enter a -1 at OutputResV.

This is fixed in 6.0.1.