Best way to free up space on I-Phone 12 mini advice please

• If you delete photos, you may not get the space back until you permanently delete them – by waiting 30 days, or by doing into the Recently Deleted album and deleting them from there. But note: once they are permanently deleted, they're gone, and there's no way of getting them back unless you made copies outside of the system.
• If you have enough iCloud space, and are using iCloud Photos, you can turn on "Optimize iPhone Storage" to see whether the phone will substitute enough "space-saving" local copies, for full-size local ones, to do any good.

Off load photos to a computer. Delete apps and data you no longer need.

If you still can't clear enough space, try updating using your computer instead of over the air.