nice that you liked this tutorial!
a) ok I will add the ‘sudo’ command to the ldconfig call
b) As far as I know the color of the nextcloud icon can’t be change. Maybe asked in the Nextcloud Forum if this is correct.
c) I don’t know if running the nextcloud client headless is possible. Maybe it is better to run to nextcloud server, which sync each other. I am pretty sure the people of the nextcloud forum know a good strategy for your problem
d) Nextcloud has sometimes problems with sync huge amount of data. Somewhere, there should be some log files, which makes the debugging easier. Again this is more a nextcloud forum topic. Sorry I can’t help in that context
All the best