Building on @pkamb answer — to shrink the accessory view into the toolbar space, you can read the placement via:

@Environment(\.tabViewBottomAccessoryPlacement) var tabViewPlacement

This returns an enum with two cases: .expanded and .inline. You can provide different views for each case, for example:

.tabViewBottomAccessory {
    switch tabViewPlacement {
    case .expanded:
        HStack {
            Spacer().frame(width: 20)
            Image(systemName: "command.square")
            Text(".tabViewBottomAccessory")
            Spacer()
            Image(systemName: "play.fill")
            Image(systemName: "forward.fill")
            Spacer().frame(width: 20)
        }

    case .inline:
        Text(".tabViewBottomAccessory")
        Image(systemName: "play.fill")
        Image(systemName: "forward.fill")
        Spacer().frame(width: 20)
    }
}

This way, you can simply omit or adjust icons in .inline mode.