dbSchema is quite nice and the price tag isn't too high I think. It's also cross-platform.
TOAD Data Modeler is also quite good (although the UI feels a bit outdated and is a bit clumsy to use, but the feature set is good). But it only runs on Windows.
pgModeler is under active development and cross platform too, but you will need to compile yourself. It is only for Postgres though (but that's enough anyway ;) )
I would recommend https://www.dbschema.com/ as it can open the individual foxpro table files, if you go for the pro version you should be able to export them to a different format (excel) that you can then use to import into another database format.
For database structure, I'd start with modeling the concepts first and thinking about performance later. Performance can be tuned later with indexes.
For database structure, start by sketching what real world concepts need to be modeled. For example: company, fiscal year, quarter, revenue, p/e value and so on. Think which piece of information is naturally an attribute of an entity (revenue in a fiscal year). Which are separate entities (company and fiscal year report). Does the entites have relationships between each other (company can have fiscal year reports)? Do they have a many-to-one or one-to-one relationship between them (company can have multiple fiscal year reports, but one fiscal year report relates to only one company)? Try to have a piece of information appear only in one place. This is called normalization in database-land.
When you've got the structure down, you should be able to draw an E/R diagram out of it. Then continue by translating the conceptual structure into SQL table definitions. Add primary key id's and foreign key relationships between tables.