Steam! Dupray Neat found on amazon. You’ll drive yourself crazy trying any other method.

I have a Bissell steam cleaner which works for everything in my home. If I need a special appliance for the pebble bathroom, I'd rather change out the tile. Makes more sense to me.

Have you consulted a tile cleaning pro?

I spoke to a few people but the room is very small job & they just didn't want to bother.

Round tiles are a terrible choice for showers. Grout is deep and water tends to "dwell" in areas and needs constant cleaning. I won't install them...only "flat" pebbles. Slope needs to be higher to properly drain.

This is really not very helpful information but thanks for your reply.

Too bad the tile cleaners aren't interested. Try a drill brush with your cordless drill to clean the grout with whatever cleaner you are supposed to use on pebbles. Then ask the tile installers what they recommend for sealing the grout after it is thoroughly dry. It may take a couple days to dry out enough to apply sealer. In other advice discussions, I have read that people with pebble tile who dry their showers with an old bathtowel for a few minutes after every use have much less cleaning to do.

Drill brushes: https://www.amazon.com/gp/product/B07F912D3F 

I actually had to replace the floor---thankfully it was a small bathroom but it just looked

so bad & no product worked. I will never use pebble flooring again.