Sounds easy but my Android 13 Motorola G54 phone purchased yesterday does not have this option:

How do I encrypt my SD card on Android?
Open your phone and go to Settings > Security.
Scroll down to find Encrypt SD Card.
Enter your password or PIN and then wait for the encryption to finish.

It does have this option:

How do I create a secure folder on my SD card?
Protect your files with Safe folder
On your Android device, open the Files by Google app .
At the bottom, tap Browse .
Scroll to "Collections."
Tap Safe folder.
Tap either PIN or Pattern. If PIN is selected: Enter your PIN. Tap Next. In the "Confirm PIN" screen, re-enter your PIN. Tap Next.

The reason I had to purchase a new phone is because when I was in Paris 5 weeks ago I either put phone down on bag on wheels while I purchased Metro tickets and walked off without picking it up or the man who squeezed past me on Metro nicked it. The latter option is better for my ego but I suspect it was the former.

The reason I only bought the phone yesterday is because I only returned from France on Friday.

I have to wait 5 business days for a replacement SIM to arrive in Perth from Sydney (for security reasons they wouldn't let me get a $2 SIM and transfer my number to that). With Xmas slowdown in mail I will probably receive it by the middle of next month. I can't access my bank accounts until I recover my phone number.

My new phone has eSIM so once my phone company starts to provide eSIMs I won't have this delay in future.

Of course, I won't ever lose phone again because I have ordered a neck lanyard.