How to get a grip on my data? File organisation. Photos. Etc

My personal data is a complete mess. It's spread over multiple external hard drives and devices, a Windows PC, iCloud, various other cloud services, many manual backups over decades. There's a lot of confusing duplication and uncertainty about whether some things are backed up at all... The photos are particularly bad. How can I get started organising all this, removing duplicates, checking that cloud services are on my local machine too, and so on? I'm fairly techie (can use Linux tools if needed) but very disorganised.