Advanced Data Protection in the UK - what options exist for client-side encryption?

The UK Government is yet again attempting to force Apple UK to build a back door into iCloud in order to allow the UK authorities to gain warranted access to users encrypted information stored in iCloud. The latest Technical Capability Notice was issued to Apple in Sept 2025 and is now restricted to UK users of iCloud, rather than worldwide. This is still an egregious over-reach of State power, and a very slippery and dangerous slope for all of us.

ADP is no longer available to new UK users. So what options are available for UK based users to implement secure client-side encryption BEFORE uploading their data to iCloud?

I have an M2 MacBook Pro running macOS Tahoe.

I've tried Cryptomator, but this regularly fails to recognise or correctly sync encrypted vaults stored in iCloud and my local machine, so this software is currently NOT a reliable option. Can anyone please recommend any alternatives that will reliably work with iCloud?