How should I make sure that I retain access to today's data 20 years from now?
If you really want to be able to keep your data that long, you need a serious plan. You need to back up everything to at least two separate devices other than your main storage, and you need to keep at least one of those devices off-site so your data can't be destroyed in a local disaster. You need to test your backups regularly to know if/when your medium is failing.
When a medium fails- or if you think it might be about to fail- get a replacement that uses more modern technology, and make a fresh copy. If you are ever about to replace your computer with a new one that can't read your old backup medium, buy newer media that does work with the new computer and make copies while you can still read the old ones. If you keep doing that regularly, you can always have a good copy that will work with your computer. It's more effort than copying to the cloud and trusting, but it means you're in control of your own data.
The real key is to keep making regular backups and regular tests. If you expect to be able to put something into a box and still use it 20 years later, you're in for an unpleasant surprise. You have to keep copying, testing, and updating your technology in order to have a serious hope of keeping up. If you do that, though, you have a very good chance of keeping access to your data at least as long as you have software that will still read it. I have 20+ year old data at work that I can still access because we've been careful about moving it to new media, and because the company that wrote the software is good about backward compatibility.