How should one advise those who have grown desensitized to sin and rely on their Muslim identity as a sense of security (for Jannah)? How can we help them develop accountability and fear of disobeying Allah ‎ﷻ?

Do not overburden them with advice. Advise them from time to time if they accept advice from you. If they do not, maybe you can send them some videos that are helpful for this case. If that does not work as well, then choose somebody they listen to and talk to that person to advise them.