I have exactly the same problem (login loop after Nvidia driver installation) and start my PC the same way as you (startx...). There is no further information about errors to be found in Nvidia and Xorg log.
At some point I found this error with "journalctl -p err -b":
Apr 07 09:41:10 xxxx login[3533]: PAM unable to dlopen(pam_lastlog.so): /usr/lib/security/pam_lastlog.so: cannot open shared object file: No such file or directory
Apr 07 09:41:10 xxxx login[3533]: PAM adding faulty module: pam_lastlog.so
Apr 07 09:41:25 xxxx login[5199]: PAM unable to dlopen(pam_lastlog.so): /usr/lib/security/pam_lastlog.so: cannot open shared object file: No such file or directory
Apr 07 09:41:25 xxxx login[5199]: PAM adding faulty module: pam_lastlog.so
The reason why it can't find /usr/lib/security/pam_lastlog.so is because there is no /secutity folder under /lib.
Since English is not my native language, I unfortunately didn't understand the solutions on Stackoverflow and co.