ᐅ Joint scraper or electric grout brush for weed removal.

Our outdoor areas are paved with Kann Vios grey paving stones measuring 40x20 cm (16x8 inches), and the terrace is made with Kann Vios slabs measuring 80x40 cm (31x16 inches).

Currently, we have two issues that are really frustrating me.

The first is the constant washing out of the basalt decorative grit from the joints. I want to address this by using Mem joint stabilizer.

Before that, I need to remove the weeds and moss from the joints, especially in the driveway. Is it worthwhile to invest in an electric joint brush, such as the Gloria Multi Brush, or would the brass brush likely scratch the paving surface, making a manual joint scraper the better option?

Use NoGrow jointing sand. That will solve the problem.
If you have a string trimmer, there are also brush attachments available.
Electric joint brushes are more like toys. A joint scraper on a handle is usually faster and easier.
Algae remover or herbicide can also help.
I would use a pressure washer to remove the gravel and then apply NoGrow sand.

But ants can get through the sand. Maybe just add more sand as well?

Nogrow sand hardens firmly after watering. However, it remains permeable to water.

A neighbor has the Nogrow sand, but I'm not really a fan of it. Various tests and videos also show this. It’s more marketing than anything else. Premium crushed sand combined with membrane joint stabilizer is ultimately also water-permeable.