Modifiable open source CRM recommendations?

Looking to replace our 15 year old CRM. I’m trying to see if there are good open source programs out there that I can modify to suit our needs.

We run both Windows and Linux servers so either OS will do.

Preferably something that works well with access or sql db’s but open to other sql platforms.

Free or one time payment is preferred over a subscription license.

And it needs to be customizable

Any thoughts? Things to try? Things to avoid?