Your real problem isn't the data format, it's the host machine - you need to be sure that the computer where your files live is still around/working in 30 years. Either make frequent backups to multiple machines, or choose a cloud provider that will be around for a while, e.g. AWS or similar (and even then, still do frequent backups to other machines).

The data format you choose should depend on the relationships present in the data. If there are relational connections between data points (e.g. data1 HAS A data2, data3 HAS MANY data4s), then SQLite is a fine choice. If all you want to do is keep the data around, there is nothing wrong with text files - perhaps the directory structure could represent the organizational structure you require.