There's sadly no silver bullet, especially for non-techies. To me it's like buying vegan products where available: I can't trivially become vegan with no effort, but I can affect market forces by preferentially buying things that align with that ideal. People could at least apply this where possible and buy products where they're in control somehow
For storage platforms like Dropbox or Onedrive, it can be as simple as ctrl+c'ing the data out of there every now and then
For Spotify, you can do the GDPR export and check that you can open it somehow. Even if it's not super readable, you can figure that out (with a tech friend or LLM perhaps) if you turn out to ever need it
For Signal, I've got no idea. The format is hard to work with for hardcore techies. They really really don't want you, ahem, attackers to get that data in the clear. (Two-sided coin). Make sure to turn on the backups, write down the passphrase, and take them off your phone every now and then, and hope there's a restore method when you need it