Apart from which wallet to choose, if you're going to keep it for 20 years, you should look into cold storage (if you haven't done it yet). A wallet on a computer connected to the internet is always at risk, and 20 years is a very long time for malware to develop. If it's online and has never been online, at least you can be sure it can't be hacked.

You don't need internet access to export private keys (assuming you're not using a webwallet). That means you can always export private keys if you keep a backup of the wallet software. It doesn't hurt to actually test this to convince yourself. And test your backups (start from scratch with a fresh offline system), to ensure you can restore your addresses from there (ideally before funding them).