Your best bet would be to transfer the files you, want to backup on an encrypted USB-stick and destroy your phone afterwards. The easiest and go-to method would be Veracrypt.
Go to this page:
https://veracrypt.fr/en/Home.html. Then to "Downloads". Download Veracrypt for your operating system. For Windows just choose "EXE Installer". Install Veracrypt accordingly. After the installation, open it and choose to "Create a volume within a partition/drive.", then select the device you need. Perhaps this could sound more complicated than it is, but you should choose a filesystem and encryption/hashing algorithm, if you don't understand what that is, it's best to choose "FAT" for your filesystem, "AES" for your encryption and "SHA-512" as your hash algorithm. For the randomness, just move your mouse back and worth randomly inside the Veracrypt window. Then you can enter your password. The password is the most important part. Make it 20 characters long, with special symbols, letters and numbers. You can mount your USB-stick whenever you want, by choosing "Mount device", selecting your device and then entering your password. Do this to check if everything worked out.
Unfortunately this is probably one of the few ways you can actually be secure from Law enforcement, if they want to check your data AND they will if you ctb. If you have questions, just ask me in this thread or via DM, I'll answer when I see it, but I can't promise that I see it, since I'm not that active here.#
(In case anyone argues, that a Linux live system with encrypted LUKS-volume would be more secure, then that's true, but probably too complicated for simply having a secure USB-stick. Walking that extra mile is probably retatively futile for OP in this case)
Can you not just put a passcode on your phone? Or are police able to get through that? I thought for Apple phones they couldn't
They are all backdoored, except for maybe some custom Android ROMs, that offer Full-Disk-Encryption, but I they're also insecure. There is no good security measure you can take to make it secure, more or less.