database versus flat files

[DeaD_EyE]

I'm currently involved in transforming large legacy "databases" (excel, access, csv...)
into something organised and searchable by the public.

Searchable by public means, that there is a web server with a database behind?
You should avoid the use of SQLite for this use case.

Try MariaDB or if you need more features, then try PostgreSQL.
Both are relational databases.