battery – Handing in macbook with root access – protecting data

There is a macbook with a badly swollen battery that will have to be replaced. The macbook itself is out of warranty, and even though the public Apple site lists an option to get it serviced out of warranty anyway, that option disappears when logging in with the associated Apple ID.

There is a Premium Reseller nearby that can replace the battery, but they require, aside from the laptop itself (obviously) full root access in the form of the password of an active administrator account.

Are there any tips for protecting the data on the laptop? Just erasing all data isn’t quite enough on an SSD as all data can easily be retrieved if I’m not mistaken. The laptop doesn’t have a T2 chip, if that makes any difference (2014 model). Are there any tips on this matter?