This is how I keep all my data secure, encrypted and in sync between 2 phones (ePhone, my Google-free phone and one iPhone), 2 laptops (Lenovo, my Linux machine and a Macbook) and one desktop (an older iMac).
A. Device Syncronization
The very first step is to keep data in sync across all devices and I do this in two stages: from mobile device to laptop and from laptop to laptop/desktop:
1. phone to laptop
iPhone to Macbook via Apple iCloud
The default iCloud account is free and offers 5GB of free space, more than enough to move data (photos, docs) from phone to laptop and vice-versa, not for long term storage and backups.
ePhone to Lenovo via Next Cloud
Same strategy with NextCloud which is the default cloud for ePhone and also offers 5GB for free.
2. between laptops/desktop via Syncthing
Syncthing is installed on all my laptops/desktop and keeps everything in sync. This is my main (and the only one) backup strategy, I do not use online clouds (except for moving data from phone to laptop), external drives or anything else. All data (photos, docs, etc) is stored on all machines.
Basically, each phone is paired with a laptop then laptops/desktop talk to each other, so my ePhone will never be connected with my Macbook directly, ePhone always talks to Lenovo laptop which is turn talks to Macbook.
NextCloud/iCloud are only used to move data between phone and laptop, once data is on laptop I move it under Syncthing to synchronize across other laptops/desktop and this is it.
B. Data Encryption
OK, we now have the data in sync between all devices, it's time to encrypt it:
This is as easy as opening System Preferences -> Security & Privacy and enable FileVault. Its file-based encryption but is a lot better than storing plain files on disk.
In Lenovo is my main workstation I have different levels of encryption:
encryption of individual files - files with sensitive content are encrypted or at least compressed and password protected
disk or partition encryption - this is what I use at the moment and I have my /home and /backups partitions encrypted using the default Linux's kernel device mapper DM-Crypt
full system encryption - planning to move to full system encryption with boot partition on removable media. Basically the system boots from an USB device which contains the passphrase/file used to decrypt and boot the entire system. No USB boot device, no party.
I suppose the default encryption is good enough, I've never messed around with this on iPhone.
Since ePhone is based on LineageOS which is in turn is based on Android it takes advantage of all Android's security/encryption without Google tracking everything.
C. Password manager
I do password management with Bitwarden and this is critical piece since an attacker gets access to all your passwords so its absolutely required to have at least 2, 3 levels of protection:
I only use Bitwarden extension for Brave browser, I never use the desktop app.
Bitwarden mobile app with finger-printing/face-recognition enabled