Magnet brackets for secret access panels - Tile Advice

[Inneedofhelpnewbie]

With fitting a concealed cistern looking at geberit, while they say maintenance can be carried out through flush plate, I was wanting to create a access panel, was going to go for a top flush and create a shelf removal, what is best make of magnetic brackets the tiles are 600 x300, wanting to also use on bath panel, one tiler just recommended I keep a few spare tiles and smash it😣, but it’s all well having a very nice bathroom if can’t access anything in case of a issue

I am wanting the area of toilet and sink, boxed out As one , with a concelead system and counter top sink of 400mm, the shelf Will be pretty wide, not sure if that makes sense but trying to make it streamlined

also is there a particular make of grout I should use for silicone of the same colour.