Nothing. To the contrary things work BETTER outside the google eco system. The way to do it is incrementally. You don't just yolo delete you Gmail day 1. I still have mine, it's just getting almost no traffic today. Start by moving to an alternative email provider. I use proton. Buy a domain so that you can move providers easily in the future and use catch all email. Do a Google takeout and store the backup somewhere safe (I just use two hard drives sitting and home, replicated). Move the thing that you need day to day somewhere else. You can pay for someone to host it for you or self host. I'm self hosting immich for my Google photos replacement. I'm using proton calendar and email for Gmail service replacements. I was already using signal for most communications, but do that. I moved to graphene to get off of android and there are some sharp edges there if you want off Google play. I had to give up Android auto and gps tends to work worse (graphene does support android auto but I didnt like the tradeoffs). Nothing dealbreaking but can be annoying.

For general security, I also use a yubikey for all services that support it, froze credit with all agencies, and added phone support passwords to all my financial institutions.

> I just use two hard drives sitting and home, replicated

The failure modes of that are fire/natural disaster, and thieves. Do that, but also have a geographically redundant backup scheme. Either encrypted eg Backblaze or a relatives house in another state.